Contents

• Context for Heightened U.S.-Iran Tensions
• Iran's Attacks on Tankers in mid-2019
• Actions by Iran's Regional Allies
• Tensions turn to Hostilities
• Iran and U.S. Downing of Drones
• UK-Iran Tensions and Iran Tanker Seizures
• Attack on Saudi Energy Infrastructure in September 2019
• U.S. Sanctions Responses to Iranian Provocations
• JCPOA-Related Iranian Responses
• Conflict Erupts (December 2019-January 2020)
• U.S. Escalation and Aftermath: Drone Strike Kills Qasem Soleimani
• Iranian Responses and Subsequent Hostilities
• Tensions Resurface in Spring 2020: Iraq and the Gulf
• Efforts to De-Escalate Tensions
• Iran-Focused Additional U.S. Military Deployments
• Gulf Maritime Security Operation
• U.S. Military Action: Options and Considerations
• Resource Implications of Military Operations
• Congressional Responses
• Possible Issues for Congress

Summary

Since May 2019, U.S.-Iran tensions have heightened significantly, and evolved into conflict after U.S. military forces killed Qasem Soleimani, the commander of the Iran's Islamic Revolutionary Guard Corps-Quds Force (IRGC-QF) and one of Iran's most important military commanders, in a U.S. airstrike in Baghdad on January 3, 2020. The United States and Iran have appeared to be on the brink of additional hostilities since, as attacks by Iran-backed groups on bases in Iraq inhabited by U.S. forces have continued.

The background to the U.S.-Iran tensions are the 2018 Trump Administration withdrawal from the 2015 multilateral nuclear agreement with Iran (Joint Comprehensive Plan of Action, JCPOA), and Iran's responses to the U.S. policy of applying "maximum pressure" on Iran. Since mid-2019, Iran and Iran-linked forces have attacked and seized commercial ships, destroyed some critical infrastructure in the Arab states of the Persian Gulf, conducted rocket and missile attacks on facilities used by U.S. military personnel in Iraq, downed a U.S. unmanned aerial vehicle, and harassed U.S. warships in the Gulf. As part of an effort it terms "maximum resistance," Iran has also reduced its compliance with the provisions of the JCPOA. The Administration has deployed additional military assets to the region to try to deter future Iranian actions.

The U.S.-Iran tensions still have the potential to escalate into all-out conflict. Iran's materiel support for armed factions throughout the region, including its provision of short-range ballistic missiles to these factions, and Iran's network of agents in Europe, Latin America, and elsewhere, give Iran the potential to expand confrontation into areas where U.S. response options might be limited. Iran has continued all its operations in the region despite wrestling with the COVID-19 pandemic that has affected Iran significantly. United States military has the capability to undertake a range of options against Iran, both against Iran directly and against its regional allies and proxies. A September 14, 2019, attack on critical energy infrastructure in Saudi Arabia demonstrated that Iran and/or its allies have the capability to cause significant damage to U.S. allies and to U.S. regional and global economic and strategic interests, and raised questions about the effectiveness of U.S. defense relations with the Gulf states.

Despite the tensions and some hostilities with Iran since 2020 began, President Donald Trump continued to state that his policy goal is to negotiate a revised JCPOA that encompasses not only nuclear issues but also Iran's ballistic missile program and Iran's support for regional armed factions. High-ranking officials from several countries have sought to mediate to try to de-escalate U.S.-Iran tensions by encouraging direct talks between Iranian and U.S. leaders. President Trump has stated that he welcomes talks with Iranian President Hassan Rouhani without preconditions, but Iran insists that the United States lift sanctions as a precondition for talks, and no U.S.-Iran talks have been known to take place to date.

Members of Congress have received additional information from the Administration about the causes of the U.S.-Iran tensions and Administration responses. They have responded in a number of ways; some Members have sought to pass legislation requiring congressional approval for any decision by the President to take military action against Iran.

Additional detail on U.S. policy options on Iran, Iran's regional and defense policy, and Iran sanctions can be found in CRS Report RL32048, Iran: Internal Politics and U.S. Policy and Options, by Kenneth Katzman; CRS Report RS20871, Iran Sanctions, by Kenneth Katzman; CRS Report R44017, Iran's Foreign and Defense Policies, by Kenneth Katzman; and CRS Report R43983, 2001 Authorization for Use of Military Force: Issues Concerning Its Continued Application, by Matthew C. Weed.

Context for Heightened U.S.-Iran Tensions

U.S.-Iran relations have been mostly adversarial since the 1979 Islamic Revolution in Iran. U.S. officials and official reports consistently identify Iran's support for militant armed factions in the Middle East region a significant threat to U.S. interests and allies. Attempting to constrain Iran's nuclear program took precedence in U.S. policy after 2002 as that program advanced. The United States also has sought to thwart Iran's purchase of new conventional weaponry and development of ballistic missiles.

In May 2018, the Trump Administration withdrew the United States from the 2015 nuclear agreement (Joint Comprehensive Plan of Action, JCPOA), asserting that the accord did not address the broad range of U.S. concerns about Iranian behavior and would not permanently preclude Iran from developing a nuclear weapon.¹ Senior Administration officials explain Administration policy as the application of "maximum pressure" on Iran's economy to (1) compel it to renegotiate the JCPOA to address the broad range of U.S. concerns and (2) deny Iran the revenue to continue to develop its strategic capabilities or intervene throughout the region.² Administration officials deny that the policy is intended to stoke economic unrest in Iran.³

As the Administration has pursued its policy of maximum pressure, including imposing sanctions beyond those in force before JCPOA went into effect in January 2016, bilateral tensions have escalated significantly. Key developments that initially heightened tensions include the following.

• On April 8, 2019, the Administration designated the Islamic Revolutionary Guard Corps (IRGC) as a Foreign Terrorist Organization (FTO),⁴ representing the first time that an official military force was designated as an FTO. The designation stated that "The IRGC continues to provide financial and other material support, training, technology transfer, advanced conventional weapons, guidance, or direction to a broad range of terrorist organizations, including Hezbollah, Palestinian terrorist groups like Hamas and Palestinian Islamic Jihad, Kata'ib Hezbollah in Iraq, al-Ashtar Brigades in Bahrain, and other terrorist groups in Syria and around the Gulf.... Iran continues to allow Al Qaeda (AQ) operatives to reside in Iran, where they have been able to move money and fighters to South Asia and Syria."⁵
• As of May 2, 2019, the Administration ended a U.S. sanctions exception for any country to purchase Iranian oil, aiming to drive Iran's oil exports to "zero."⁶
• Since May 2019, the Administration has ended five out of the seven waivers under the Iran Freedom and Counter-Proliferation Act (IFCA, P.L. 112-239)—waivers that allow countries to help Iran remain within limits set by the JCPOA.⁷
• On May 5, 2019, citing reports that Iran or its allies might be preparing to attack U.S. personnel or installations, then-National Security Adviser John Bolton announced that the United States was accelerating the previously planned deployment of the USS Abraham Lincoln Carrier Strike Group and sending a bomber task force to the Persian Gulf region.⁸
• On May 24, 2019, the Trump Administration notified Congress of immediate foreign military sales and proposed export licenses for direct commercial sales of defense articles—training, equipment, and weapons—with a possible value of more than $8 billion, including sales of precision guided munitions (PGMs) to Saudi Arabia and the United Arab Emirates (UAE). In making the 22 emergency sale notifications, Secretary of State Pompeo invoked emergency authority codified in the Arms Export Control Act (AECA), and cited the need "to deter further Iranian adventurism in the Gulf and throughout the Middle East."⁹

Iran's Attacks on Tankers in mid-2019

Iran responded to the additional steps in the U.S. maximum pressure campaign in part by demonstrating its ability to harm global commerce and other U.S. interests and to raise renewed concerns about Iran's nuclear activities. Iran apparently has sought to cause international actors, including those that depend on stable oil supplies, to put pressure on the Trump Administration to reduce its sanctions pressure on Iran.

• On May 12-13, 2019, four oil tankers—two Saudi, one Emirati, and one Norwegian ship—were damaged. Iran denied involvement, but a Defense Department (DOD) official on May 24, 2019, attributed the tanker attacks to the IRGC.¹⁰ A report to the United Nations based on Saudi, UAE, and Norwegian information found that a "state actor" was likely responsible, but did not name a specific perpetrator.¹¹
• On June 13, 2019, two Saudi tankers in the Gulf of Oman were attacked. Secretary of State Michael Pompeo stated, "It is the assessment of the U.S. government that Iran is responsible for the attacks that occurred in the Gulf of Oman today….based on the intelligence, the weapons used, the level of expertise needed to execute the operation, recent similar Iranian attacks on shipping, and the fact that no proxy group in the area has the resources and proficiency to act with such a high degree of sophistication.... "¹²

Figure 1. Selected Iran-supported Groups

Source: "Iran Military Power: Ensuring Regime Survival and Securing Regional Dominance," Defense Intelligence Agency (DIA), November 2019.

Actions by Iran's Regional Allies

Iran's allies in the region have been conducting attacks that might be linked to U.S.-Iran tensions, although it is not known definitively whether Iran directed or encouraged each attack (see Figure 1 for a map of Iran-supported groups). Trump Administration officials, particularly Secretary of State Pompeo, has stated that the United States will hold Tehran responsible for the actions of its regional allies.¹³ Some of the most significant actions by Iran-linked forces during mid-2019 are the following:

• On May 19, 2019, a rocket was fired into the secure "Green Zone" in Baghdad but it caused no injuries or damage.¹⁴ Iran-backed Iraqi militias were widely suspected of the firing and U.S. Defense Department officials attributed it to Iran.¹⁵ The incident came four days after the State Department ordered "nonemergency U.S. government employees" to leave U.S. diplomatic facilities in Iraq, claiming a heightened threat from Iranian allies. An additional rocket attack launched from Iraq included a May 2019 attack on Saudi pipeline infrastructure in Saudi Arabia with an unmanned aerial aircraft, first considered to have been launched from Yemen.¹⁶ Further attacks, discussed below, have led to U.S.-Iran hostilities.
• In June 2019 and periodically thereafter, the Houthis, who have been fighting against a Saudi-led Arab coalition that intervened in Yemen against the Houthis in March 2015, claimed responsibility for attacks on an airport in Abha, in southern Saudi Arabia,¹⁷ and on Saudi energy installations and targets. The Houthis claimed responsibility for the large-scale attack on Saudi energy infrastructure on September 14, 2019, but, as discussed below, U.S. and Saudi officials have concluded that the attack did not originate from Yemen.
• In a June 13, 2019, statement, Secretary of State Pompeo asserted Iranian responsibility for a May 31, 2019, car bombing in Afghanistan that wounded four U.S. military personnel. Administration reports have asserted that Iran was providing materiel support to some Taliban militants, but outside experts asserted that the Iranian role in that attack is unlikely.¹⁸

Tensions turn to Hostilities

In subsequent weeks, U.S.-Iran tensions erupted into direct hostilities as well as further Iranian actions against U.S. partners.

Iran and U.S. Downing of Drones

On June 20, 2019, Iran shot down an unmanned aerial surveillance aircraft (RQ-4A Global Hawk Unmanned Aerial Vehicle) near the Strait of Hormuz, claiming it had entered Iranian airspace over the Gulf of Oman. U.S. Central Command officials stated that the drone was over international waters.¹⁹ Later that day, according to his posts on the Twitter social media site, President Trump ordered a strike on three Iranian sites related to the Global Hawk downing, but called off the strike on the grounds that it would have caused Iranian casualties and therefore been "disproportionate" to the Iranian shoot down.²⁰ The United States did reportedly launch a cyberattack against Iranian equipment used to track commercial ships.²¹ On July 18, 2019, President Trump announced that U.S. forces in the Gulf had downed an Iranian drone via electronic jamming in "defensive action" over the Strait of Hormuz (see Figure 3). Iran denied that any of its drones were shot down.

UK-Iran Tensions and Iran Tanker Seizures

U.S.-Iran tensions spilled over into confrontations between Iran and the UK. On July 4, 2019, authorities from the British Overseas Territory Gibraltar, backed by British marines, impounded an Iranian tanker, the Grace I, off the coast of Gibraltar for allegedly violating an EU embargo on the provision of oil to Syria. Iranian officials termed the seizure an act of piracy, and in subsequent days, the IRGC Navy sought to intercept a UK-owned tanker in the Gulf, the British Heritage, but the force was reportedly driven off by a British warship. On July 19, the IRGC Navy seized a British-flagged tanker near the Strait of Hormuz, the Stena Impero, claiming variously that it violated Iranian waters, was polluting the Gulf, collided with an Iranian vessel, or that the seizure was retribution for the seizure of the Grace I.

On July 22, 2019, the UK's then-Foreign Secretary Jeremy Hunt explained the government's reaction to the Stena Impero seizure as pursuing diplomacy with Iran to peacefully resolve the dispute, while at the same time sending additional naval vessels to the Gulf to help secure UK commercial shipping. On August 15, 2019, following a reported pledge by Iran not to deliver the oil cargo to Syria, a Gibraltar court ordered the ship (renamed the Adrian Darya 1) released. Gibraltar courts turned down a U.S. Justice Department request to impound the ship as a violator of U.S. sanctions on Syria and on the IRGC, which the U.S. filing said was financially involved in the tanker and its cargo.²² The ship apparently delivered its oil to Syria despite the pledge²³ and, as a consequence, the United States imposed new sanctions on individuals and entities linked to the ship and to the IRGC. On September 22, 2019, Iran released the Stena Impero.

Separate from the UK-Iran dispute over the Grace I and the Stena Impero, Iran seized an Iraqi tanker on August 5, 2019, for allegedly smuggling Iranian diesel fuel to "Persian Gulf Arab states."²⁴

Attack on Saudi Energy Infrastructure in September 2019²⁶

Iran appeared to escalate tensions significantly by conducting an attack, on September 14, 2019, on multiple locations within critical Saudi energy infrastructure sites at Khurais and Abqaiq. The Houthi movement in Yemen, which receives arms and other support from Iran, claimed responsibility but Secretary of State Pompeo stated "Amid all the calls for de-escalation, Iran has now launched an unprecedented attack on the world's energy supply. There is no evidence the attacks came from Yemen."²⁷ Press reports stated that U.S. intelligence indicates that Iran itself was the staging ground for the attacks, in which cruise missiles, possibly assisted by unmanned aerial vehicles, struck nearly 20 targets at those Saudi sites.²⁸ Iranian officials denied responsibility for the attack.

The attack shut down a significant portion of Saudi oil production and, whether conducted by Iran itself or by one of its regional allies, escalated U.S.-Iran and Iran-Saudi tensions and demonstrated a significant capability to threaten U.S. allies and interests. President Trump stated on September 16, 2019, that he would "like to avoid" conflict with Iran and the Administration did not retaliate militarily. U.S. officials did announce modest increases in U.S. forces in the region and some new U.S. sanctions on Iran.

The attacks on the Saudi infrastructure raised several broad questions, including

• What is the extent and durability of the long-standing implicit and explicit U.S. security guarantees to the Gulf states?
• Have Iran's military technology capabilities advanced further than has been estimated by U.S. officials and the U.S. intelligence community?

U.S. Sanctions Responses to Iranian Provocations

As tensions with Iran increased, the Trump Administration increased economic pressure on Iran to weaken it strategically, and compel it to negotiate a broader resolution of U.S.-Iran differences.

• On May 8, 2019, the President issued Executive Order 13871, blocking U.S.-based property of persons and entities determined to have conducted significant transactions with Iran's iron, steel, aluminum, or copper sectors.²⁹
• On June 24, 2019, President Trump issued Executive Order 13876, blocking the U.S.-based property of Supreme Leader Ali Khamene'i and his top associates. Sanctions on several senior officials, including Iran's Foreign Minister Mohammad Javad Zarif, have since been imposed under that Order.
• On September 4, 2019, the State Department Special Representative for Iran and Senior Advisor to the Secretary of State Brian Hook said the United States would offer up to $15 million to any person who helps the United States disrupt the financial operations of the IRGC and its Qods Force—the IRGC unit that assists Iran-linked forces and factions in the region. The funds are to be drawn from the long-standing "Rewards for Justice Program" that provides incentives for persons to help prevent acts of terrorism.
• On September 20, 2019, the Trump Administration imposed additional sanctions on Iran's Central Bank by designating it a terrorism supporting entity under Executive Order 13224. The Central Bank was already subject to a number of U.S. sanctions, rendering unclear whether any new effect on the Bank's ability to operate would result. Also sanctioned was an Iranian sovereign wealth fund, the National Development Fund of Iran.
• In early 2020, U.S. officials indicated that they would use all available options to achieve an extension of the arms transfer ban on Iran provided by U.N. Security Council Resolution 2231, and which expires on October 18, 2020. U.S. officials insisted that the ban be extended in order to prohibit Russia and China from proceeding with planned arms sales to Iran, which would have the effect of increasing the conventional military threat from Iran. See CRS In Focus IF11429, U.N. Ban on Iran Arms Transfers, by Kenneth Katzman.

JCPOA-Related Iranian Responses³⁰

Since the Trump Administration's May 2018 announcement that the United States would no longer participate in the JCPOA, Iranian officials repeatedly have rejected renegotiating the agreement or discussing a new agreement. Tehran also has conditioned its ongoing adherence to the JCPOA on receiving the agreement's benefits from the remaining JCPOA parties, collectively known as the "P4+1." On May 10, 2018, Iranian Foreign Minister Mohammad Javad Zarif wrote that, in order for the agreement to survive, "the remaining JCPOA Participants and the international community need to fully ensure that Iran is compensated unconditionally through appropriate national, regional and global measures." He added that

Iran has decided to resort to the JCPOA mechanism [the Joint Commission established by the agreement] in good faith to find solutions in order to rectify the United States' multiple cases of significant non-performance and its unlawful withdrawal, and to determine whether and how the remaining JCPOA Participants and other economic partners can ensure the full benefits that the Iranian people are entitled to derive from this global diplomatic achievement.

Tehran also threatened to reconstitute and resume the country's pre-JCPOA nuclear activities.

Several meetings of the JCPOA-established Joint Commission since the U.S. withdrawal have not produced a firm Iranian commitment to the agreement. Tehran has argued that the remaining JCPOA participants' efforts have been inadequate to sustain the agreement's benefits for Iran. In May 8, 2019, letters to the other JCPOA participant governments, Iran announced that, as of that day, Tehran had stopped "some of its measures under the JCPOA," though the government emphasized that it was not withdrawing from the agreement. Specifically, Iranian officials said that the government will not transfer low enriched uranium (LEU) or heavy water out of the country in order to maintain those stockpiles below the JCPOA-mandated limits. A May 8, 2019, statement from Iran's Supreme National Security Council explained that Iran "does not anymore see itself committed to respecting" the JCPOA-mandated limits on LEU and heavy water stockpiles.

Beginning in July 2019, the International Atomic Energy Agency (IAEA) verified that some of Iran's nuclear activities were exceeding JCPOA-mandated limits; the Iranian government has since increased the number of such activities. Specifically, according to IAEA reports, Iran has exceeded JCPOA-mandated limits on its heavy water stockpile, the number of installed centrifuges in Iran's pilot enrichment facility, Iran's LEU stockpile, and the LEU's concentration of the relevant fissile isotope uranium-235. In addition, Tehran is conducting JCPOA-prohibited research and development activities, as well as centrifuge manufacturing, and has also begun to enrich uranium at its Fordow enrichment facility.

The Iranian government announced in a January 5, 2020, statement "the fifth and final step in reducing" Tehran's JCPOA commitments, explaining that Tehran would "set aside the final operational restrictions under the JCPOA which is 'the restriction on the number of centrifuges.' " ³¹ The statement provided no details regarding concrete changes to Iran's nuclear program, but the term "restrictions" may refer to the JCPOA-mandated limits on installed centrifuges at the country's commercial enrichment facility. According to a March report from the IAEA Director General., Iran has not exceeded these limits.³² The January 5 announcement added that "[i]n case of the removal of sanctions and Iran benefiting from the JCPOA," Iran "is ready to resume its commitments" pursuant to the agreement.³³ In a May 6 speech, Iranian President Hassan Rouhani characterized Tehran's aforementioned actions as a withdrawal from the government's JCPOA commitments "in an equal scale," Whenever the United States and P4+1 "are ready to observe their full commitments under the JCPOA," Iran "will return to the JCPOA the same day,"³⁴ he added. According to an article published May 6, Iran's Permanent Representative to the IAEA Kazzem Gharibabdi stated that Iran could reduce or end its cooperation with the IAEA if the United States and P4+1 continue actions which, Tehran argues, damage the JCPOA.³⁵

Conflict Erupts (December 2019-January 2020)

In early December 2019, press reports and U.S. officials indicated that Iran was supplying short- range missiles to allied forces inside Iraq.³⁶ A series of indirect fire attacks in mid-December 2019 targeted Iraqi military facilities where U.S. forces are co-located.³⁷ In response, Secretary Pompeo issued a statement saying, "We must also use this as an opportunity to remind Iran's leaders that any attacks by them, or their proxies of any kind, that harm Americans, our allies, or our interests will be answered with a decisive U.S. response."³⁸ Secretary of Defense Mark Esper stated that he urged then-Iraqi Prime Minister Adel Abd Al Mahdi to "take proactive actions…to get that under control."³⁹

On December 27, 2019, a rocket attack on a base near Kirkuk in northern Iraq killed a U.S. contractor and wounded four U.S. service members and two Iraqi service members. Two days later, the U.S. launched retaliatory airstrikes on five facilities (three in Iraq, two in Syria) used by the Iran-backed Iraqi armed group Kata'ib Hezbollah (KH), a U.S.-designated Foreign Terrorist Organization to which the U.S. attributed the attack. KH leader and leading figure in the Iraqi-state affiliated Popular Mobilization Forces Abu Mahdi al Muhandis said dozens of fighters were killed and injured and promised a "very tough response" on U.S. forces in Iraq.⁴⁰

Iraqi leaders, including those who want to maintain good relations with both the United States and Iran, criticized the strikes as a "violation of Iraqi sovereignty."⁴¹ The hostilities came as Iran sought to preserve its political influence amidst large-scale demonstrations in which hundreds of protestors were killed by security forces⁴²and which contributed to Abd Al Mahdi's resignation that month. He continues to serve in a caretaker role while Iraqi political leaders negotiate a transition. In a December 6, 2019 press briefing announcing sanctions designations of several Iran-linked Iraqi groups and individuals, Assistant Secretary of State for Near Eastern Affairs David Schenker said

the United States Government will work with anyone in the Iraqi Government who is willing to put Iraqi interests first.... This is a sine qua non. But we see in the process of establishing a new government or determining who the next prime minister will be that [IRGC-QF commander] Qasem Soleimani is in Baghdad working this issue. It seems to us that foreign terrorist leaders, or military leaders, should not be meeting with Iraqi political leaders to determine the next premier of Iraq, and this is exactly what the Secretary says about being perhaps the textbook example of why Iran does not behave and is not a normal state. This is not normal. This is not reasonable. This is unorthodox and it is incredibly problematic, and it is a huge violation of Iraqi sovereignty.⁴³

On December 31, 2019, two days after the U.S. airstrikes against KH targets in Iraq and Syria, supporters of KH and other Iran-backed Iraqi militias surrounded and then entered the U.S. Embassy in Baghdad, setting some outer buildings on fire. The militiamen withdrew after their leaders said they obtained acting Prime Minister Abdul Mahdi's promise for "serious work" on a parliamentary vote to expel U.S. forces from the country, a long-sought goal of Iran and its Iraqi allies.⁴⁴ President Trump tweeted that Iran, which "orchestrat[ed the] attack," would "be held fully responsible for lives lost, or damage incurred, at any of our facilities. They will pay a very BIG PRICE!"⁴⁵

U.S. Escalation and Aftermath: Drone Strike Kills Qasem Soleimani

On January 3, 2020, Iraq time, a U.S. military armed drone strike killed IRGC-QF Commander Major General Qasem Soleimani in what the Defense Department termed a "defensive action." The statement cited Soleimani's responsibility for "the deaths of hundreds of Americans and coalition service members" and his approval of the Embassy blockade, and stated that he was "actively developing plans to attack American diplomats and service members in Iraq and throughout the region." The strike, conducted while Soleimani was leaving Baghdad International Airport, also killed KH leader Abu Mahdi al-Muhandis, who also headed the broader Popular Mobilization Forces (PMF) made up mostly of militia fighters, and other Iranian and Iraqi figures. Iraq's Council of Representatives (CoR) on January 5, 2020, voted to direct the government "to work towards ending the presence of all foreign troops on Iraqi soil," according to the media office of the Iraqi Parliament.⁴⁶

Soleimani was widely regarded as one of the most powerful and influential figures in Iran, with a direct channel to Khamene'i, who serves as Commander-in-Chief of all Iranian armed forces.⁴⁷ One expert described him as "the military center of gravity of Iran's regional hegemonic efforts" and "an operational and organization genius who likely has no peer in the upper ranks of the Islamic Revolutionary Guard Corps."⁴⁸ Others contend that "he was only the agent of a government policy that preceded him and will continue without him."⁴⁹

Iranian Responses and Subsequent Hostilities

Secretary of State Pompeo underscored that the United States is not seeking further escalation, but Iran's leaders, including Supreme Leader Ali Khamene'i, threatened to retaliate for the Soleimani killing. That retaliation, codenamed "Operation Martyr Soleimani" came on January 8, 2020, in the form of an Iranian ballistic missile strike on two Iraqi bases – Ayn al-Asad in western Iraq and an airbase near Irbil, in Kurdish-controlled northern Iraq. The United States reported no "casualties," according to a statement by President Trump on January 8, 2020, and the United States reportedly had some advanced warning of the attack, via Iraqi officials. The President added that "Iran appears to be standing down, which is a good thing for all parties concerned and a very good thing for the world," and there was no U.S. military retaliation for Iran's missile strike.⁵⁰ Still, over the coming weeks, about 110 U.S. military personnel were diagnosed with various forms of traumatic brain injury, mostly concussions from the blast.

Iran's ability to hit Ayn al-Asad with some degree of precision indicated growing capability in Iran's missile capabilities. For the past several years, the U.S. intelligence community, in its annual worldwide threat assessment briefings for Congress, has assessed that Iran has "the largest inventory of ballistic missiles in the region,"⁵¹ and the 2019 version of the annual, congressionally-mandated report on Iran's military power by the Defense Intelligence Agency indicated that Iran is advancing its drone technology and the precision targeting of the missiles it provides to its regional allies.⁵² Israel asserts that these advances pose a sufficient threat to justify Israeli attacks against Iranian and Iran-allied targets in the region, including in Lebanon, Syria, and Iraq.⁵³

Tensions Resurface in Spring 2020: Iraq and the Gulf

After about two months marked only by casualty-free occasional rocket attacks in Iraq by Iran-backed factions, U.S.-Iran tensions began to rise again in March 2020. On March 11, 2020, a rocket attack on Camp Taji in Iraq, allegedly by KH, killed two U.S. military personnel and one British medic serving with the U.S.-backed coalition fighting the Islamic State organization. On March 13, 2020, the commander of U.S. Central Command (CENTCOM), Gen. Kenneth McKenzie, said the United State used manned aircraft to strike several sites near Baghdad that KH uses as storage areas for advanced conventional weapons, heavy rockets, and associated propellant. According to McKenzie: "We also assessed that the destruction of these sites will degrade Kata'ib Hezbollah's ability to conduct future strikes."⁵⁴

However, the deterrent effect of the U.S. strikes appear limited. On March 15, 2020, according to the Defense Department, three U.S. service personnel were injured in another rocket attack on the same location, Camp Taji, of which two were seriously wounded. Some Iraqi military personnel were also wounded. The United States did not retaliate.

The new hostilities in Iraq came amid Iraq's struggles to establish a government to succeed that of Adel Abdul Mahdi, who remains a caretaker prime minister. Soleimani's successor, Esmail Qaani, made his first reported visit to Iraq in late March, reportedly in an effort to unite Iran-backed factions on a successor to Abdul Mahdi. The Iraqi political struggles to form a new government reflect the continuing Iranian and U.S. effort to limit each other's influence on Iraqi politics.

Several weeks after the Iraq rocket attacks, Iran resumed some provocations in the Persian Gulf. On April 14, 2020, the IRGC Navy forcibly boarded and steered into Iranian waters a Hong Kong-flagged tanker. The next day, eleven IRGC Navy small boats engaged in what the State Department called "high speed, harassing approaches" of five U.S. naval vessels conducting routine exercises in the Gulf."⁵⁵ The United States, either separately or as part of the IMSC Gulf security mission discussed above, did not respond militarily to the Iranian actions. However, on April 22, President Trump posted a message on Twitter saying: "I have instructed the United States Navy to shoot down and destroy any and all Iranian gunboats if they harass our ships at sea." U.S. defense officials characterized the President's message as a warning Iran against further such actions, but they stressed that U.S. commanders have discretion about how to respond to future provocative actions by Iran.

Also on April 22, the IRGC announced that it had launched a "military satellite" into orbit. Secretary of State Pompeo reacted by stating "I think today's launch proves what we've been saying all along here in the United States [that Iran's space launches are not for purely commercial purposes]."⁵⁶ On May 6, 2020, the Chairman of the Joint Chiefs of Staff Gen. Mark Milley stated "Well, let me put it this way, they launched a satellite vehicle, I think we publicly had stated it was tumbling. So the satellite itself, not overly concerned about it, but the missile technology, the secondary and second and third order missile technology and the lesson learned from that, that is a concern because, you know, different missiles can do different things and one can carry a satellite, another can carry some sort of device that can explode. So, the bottom line is yes, it is a security concern any time Iran is testing any type of long-range missile."⁵⁷

Efforts to De-Escalate Tensions

U.S. partner countries and U.N. officials have consistently called for the de-escalation of tensions and the avoidance of war. The EU countries have refused to join the U.S. maximum pressure campaign as a consequence of Iran's provocative acts, although the UK, France, and Germany have urged Iran to negotiate a new JCPOA that includes limits on Iran's missile development.⁵⁸ Some U.S. allies have joined a U.S. effort to deter Iran from further attacks on shipping in the Gulf. EU officials have said that they still hope to preserve the JCPOA could be preserved.⁵⁹

The United States and Iran do not have diplomatic relations and there have been no known high-level talks between Iran and Administration officials since the Trump Administration withdrew from the JCPOA. Prior to the Soleimani killing, various third country leaders, such as Japanese Prime Minister Shinzo Abe in mid-2019 and again in a visit to Iran in December 2019, have sought to move Tehran and Washington toward direct talks.

Several Gulf countries have sent delegations to Iran to try to ease U.S.-Iran tensions that the Gulf leaders say could lead to severe destruction in the Gulf states themselves in the event of conflict.⁶⁰ A UAE delegation that visited Tehran in late July 2019 undertook the first UAE security talks with Iran since 2013. In late 2019, Saudi Arabia reportedly sought help from Pakistan and Iraq in undertaking talks with Iran to lower tensions.⁶¹

In August 2019, French President Macron appeared to make progress but ultimately did not produce U.S.-Iran talks. While hosting the G-7 summit in Biarritz, Macron invited Foreign Minister Zarif to meet with him there. No Trump-Zarif meeting took place in Biarritz but, at a press conference at the close of the summit, President Trump reiterated his willingness, in principle, to meet with Iranian President Hassan Rouhani, presumably during the U.N. General Assembly meetings in New York in September. President Trump reportedly considered supporting a French proposal to provide Iran with a credit line as an incentive for Iran to meet with him.⁶² However, in the wake of the September 14, 2019 attacks in Saudi Arabia and since, the Supreme Leader has stated that there would be no U.S.-Iran talks and Rouhani and Zarif have since repeatedly restated the view that U.S. sanctions be lifted before any such talks.

Iran-Focused Additional U.S. Military Deployments

For the stated purpose of trying to deter further Iranian attacks and protecting U.S. forces already in the region, the United States added forces and military capabilities in the region. As of early 2020, approximately 14,000 U.S. military personnel had been added to a baseline of more than 60,000 U.S. forces in and around the Persian Gulf, which include those stationed at military facilities in the Arab states of the Gulf Cooperation Council (GCC: Saudi Arabia, Kuwait, UAE, Qatar, Oman, and Bahrain), and those in Iraq and Afghanistan.⁶³ Defense Department officials indicated that the additional deployments mostly restored forces who were redeployed from the region a few years ago, and did not represent preparation for any U.S. offensive against Iran.⁶⁴

Among the additional deployments, the United States sent additional Patriot and Terminal High Altitude Area Defense (THAAD) missile defense systems in the region. ⁶⁵ Some of the additional forces sent deployed to Prince Sultan Air Base in Saudi Arabia, which is south of Riyadh. ⁶⁶ U.S. forces used the base to enforce a no-fly zone over southern Iraq during the 1990s, but left there after Saddam Hussein was ousted by Operation Iraqi Freedom in 2003.

As 2020 progressed, some U.S. deployments changed. In March 2020, hundreds of U.S forces in Iraq were redeployed from smaller bases in Iraq to larger ones, and some were withdrawn to locations elsewhere in the region. The redeployments reportedly were due to a waning threat in Iraq from the Islamic State organization as well as the apparent need to better defend U.S. forces from attacks by Iran-backed militias.⁶⁷ In early May 2019, it was reported that the United States had withdrawn some Patriot air defenses and combat aircraft from Saudi Arabia and other locations in the Gulf, although U.S. officials denied that the deployments signaled an altered assessment of the Iran threat or would degrade U.S. capabilities to deter Iran.⁶⁸

Gulf Maritime Security Operation

Iran's naval actions in the Gulf in mid-2019 prompted the formation of a new, U.S.-led military operation to protect commercial shipping in the Gulf. The maritime security and monitoring initiative for the Gulf, the Bab el-Mandeb Strait, and the Suez Canal was termed "Operation Sentinel." Operation Sentinel began activities in August 2019 and was then formally inaugurated as the International Maritime Security Construct (IMSC) in Bahrain in November 2019. It consists of: the United States, the UK, the UAE, Saudi Arabia, Bahrain, Qatar, Kuwait, Albania, and Australia) operating four sentry ships at crucial points in the Gulf.⁶⁹ Additionally, Israeli Foreign Minister Yisrael Katz said Israel would join the coalition, but Defense Department officials have not listed Israel as a participant in IMSC to date. China's ambassador to the UAE said in early August 2019 that China was considering joining the mission, although no announcement of China's participation has since been made. The IMSC supplements longstanding multilateral Gulf naval operations that have targeted smuggling, piracy, the movement of terrorists and weaponry, and other potential threats in the Gulf.

Other countries have started separate maritime security missions in the Gulf. France leads a maritime security mission, headquartered in Abu Dhabi, that began activities in early 2020. India has sent some naval vessels to the Gulf to protect Indian commercial ships. In December 2019, Japan sent vessels to protect Japanese shipping, also separate from the IMSC.

U.S. Military Action: Options and Considerations

The military is a tool of national power that the United States can use to advance its objectives, and the design of a military campaign and effective military options depend on the policy goals that U.S. leaders seek to accomplish. The Trump Administration has stated that its "core objective ... is the systemic change in the Islamic Republic's hostile and destabilizing actions, including blocking all paths to a nuclear weapon and exporting terrorism."⁷⁰As such, the military could be used in a variety of ways to try to contain and dissuade Iran from prosecuting its "hostile and destabilizing actions." These ways range from further increasing presence and posture in the region to use of force to change Iran's regime. As with any use of the military instrument of national power, any employment of U.S. forces in this scenario could result in further escalation of a crisis.

U.S. military action may not be the appropriate tool to achieve systemic change within the Iranian regime, and may potentially set back the political prospects of Iranians sympathetic to a change of regime. Some observers question the utility of military power against Iran due to global strategic considerations. The 2017 National Security Strategy and 2018 National Defense Strategy both noted that China and Russia represent the key current and future strategic challenges to the United States. As such, shifting additional military assets into the United States Central Command (CENTCOM) area of responsibility requires diverting them from use in other theaters such as Europe and the Pacific, thereby sacrificing other long-term U.S. strategic priorities.

Secretary of Defense Mark Esper and other U.S. officials have stated that the additional U.S. deployments since May 2019 are intended to deter Iran from taking any further provocative actions and position the United States to defend U.S. forces and interests in the region.⁷¹ Iranian attacks after previous U.S. deployments could suggest that deploying additional assets and capabilities might not necessarily succeed in deterring Iran from using military force.

On the other hand, there are risks to military inaction that might potentially outweigh those associated with the employment of force. For example, should Iran acquire a nuclear weapons capability, U.S. options to contain and dissuade it from prosecuting hostile activities could be significantly more constrained than they are at present.⁷²

For illustrative purposes only, below are some potential additional policy options related to the possible use of military capabilities against Iran. Not all of these options are mutually exclusive, nor do they represent a complete list of possible options, implications, and risks. Congress has assessed its role in any decisions regarding whether to undertake military action against Iran, as discussed later in this report. The following discussion is based entirely on open-source materials.

• Operations against Iranian allies or proxies. The Administration might decide to take additional action against Iran's allies or proxies, such as Iran-backed militias in Iraq, Lebanese Hezbollah, or the Houthi movement in Yemen. Such action could take the form of air operations, ground operations, special operations, or cyber and electronic warfare. Further attacks on Iranian allies could be intended to seriously degrade the military ability of the Iranian ally in question and undertaken by U.S. forces, partner government forces, or both. At the same time, military action against Iran's allies could harm the prospects for resolution of the regional conflicts in which Iranian allies operate.
• Retaliatory Action against Iranian Key Targets and Facilities. The United States retains the option to undertake air and missile strikes, as well as special operations and cyber and electronic warfare against Iranian targets, such as IRGC Navy vessels in the Gulf, nuclear facilities, military bases, ports, oil installations, and any number of other targets within Iran itself.⁷³ Iran's major Gulf ports are shown in Figure 2.
• Blockade. Another option could be to establish a naval and/or air quarantine of Iran. Iran has periodically, including since mid-2019, threatened to block the vital Strait of Hormuz. Some observers have in past confrontations raised the prospect of a U.S. closure of the Strait or other waterways to Iranian commerce.⁷⁴ Under international law, blockades are acts of war.
• Invasion. Although apparently far from current consideration because of the potential risks and costs, a U.S. invasion of Iran to oust its regime is among the options. Press reports in May 2019 indicated that the Administration was considering adding more than 100,000 military forces to the Gulf to deter Iran from any attacks.⁷⁵ Such an option, if exercised, might be interpreted as potentially enhancing the U.S. ability to conduct ground attacks inside Iran, although military experts have indicated that a U.S. invasion and/or occupation of Iran would require many more U.S. forces than those cited.⁷⁶ Iran's population is about 80 million, and its armed forces collectively number about 525,000, including 350,000 regular military and 125,000 IRGC forces.⁷⁷ There has been significant antigovernment unrest in Iran over the past 10 years, but there is no indication that there is substantial support inside Iran for a U.S. invasion to change Iran's regime.

Resource Implications of Military Operations

Without a more detailed articulation of how the military might be employed to accomplish U.S. objectives vis-a-vis Iran, and a reasonable level of confidence about how any conflict might proceed, it is difficult to assess with any precision the likely fiscal costs of a military campaign, or even just heightened presence. Still, any course of action listed in this report is likely to incur significant additional costs. Factors that might influence the level of expenditure required to conduct operations include, but are not limited to, the following:

• The number of additional forces, and associated equipment, deployed to the Persian Gulf or the CENTCOM theater more broadly. In particular, deploying forces and equipment from the continental United States (if required) would likely add to the costs of such an operation due to the logistical requirements of moving troops and materiel.
• The mission set that U.S. forces are required to prosecute and its associated intensity. Some options leading to an increase of the U.S. posture in the Persian Gulf might require upgrading existing facilities or new construction of facilities and installations. By contrast, options that require the prosecution of combat operations would likely result in significant supplemental and/or overseas contingency operations requests, particularly if U.S. forces are involved in ground combat or post-conflict stabilization operations.
• The time required to accomplish U.S. objectives. As demonstrated by operations in Iraq and Afghanistan, the period of anticipated involvement in a contingency is a critical basis for any cost analysis. On one hand, a large stabilizing or occupying ground force to perform stabilization and reconstruction operations, for example, would likely require the expenditure of significant U.S. resources.

At the same time, there is potential for some U.S. costs to be offset by contributions. The Persian Gulf states and other countries have a track record of offsetting U.S. costs for Gulf security. In the current context, President Trump stated in October 2019 that Saudi Arabia would pay for the deployment of additional U.S. troops and capabilities to assist with the territorial defense of Saudi Arabia and the deterrence of Iranian aggression in the region overall, and subsequent reports indicate that U.S. and Saudi officials are negotiating a cost-sharing arrangement for the new deployments.⁷⁸

Congressional Responses

Members of Congress have responded in different ways to tensions with Iran and to related questions of authorization for the use of military force.

Various instances of increased U.S.-Iran tensions in the past year have prompted some Members to express concern about or support for potential military operations against Iran. These episodes include the June 2019 attacks against tankers in the Gulf of Oman and Iran's shoot down of a U.S. military drone; the September 2019 attacks on Saudi oil facilities at Abqaiq and Khurais; and the buildup of U.S. forces in the region in response to Iranian activities.

Throughout this period, Congress passed legislation with provisions specifying that authorization for the use of force against Iran is not granted. For instance, Section 1284 of the FY2020 NDAA (P.L. 116-92 , December 2019) states that "Nothing in this Act, or any amendment made by this Act, may be construed to authorize the use of military force, including the use of military force against Iran or any other country." Similarly, Section 9024 of Division A of H.R. 1158, the Consolidated Appropriations Act, 2020, (P.L. 116-89 , December 2019) states that "Nothing in this Act may be construed as authorizing the use of force against Iran."

However, Congress has not prohibited the use of funds for operations against Iran, despite the introduction of several standalone measures that would do so, such as the Prevention of Unconstitutional War with Iran Act of 2019 (H.R. 2354/S. 1039).While the House did pass legislation that included a prohibition on funding for the use of force against Iran, including Section 1229 of H.R. 2500, the National Defense Authorization Act (NDAA) for FY2020, the Senate rejected by a 50-40 vote an amendment (S.Amdt. 883) that would have added similar text to its version of the FY2020 NDAA, and the House-passed language was not included in conference text of the bill.

In response to these moves, President Trump stated that he had wide-ranging authority to unilaterally initiate the use of military force, as successive Administrations have maintained.⁷⁹ For instance, in a June 24 interview, President Trump reiterated that he believed he had the authority to order military action against Iran without congressional approval, adding, "I do like keeping them [Congress] abreast, but I don't have to do it, legally."⁸⁰ Secretary Pompeo suggested in an April 2019 hearing that the 2001 authorization for use of military force (AUMF, P.L. 107-40) against those responsible for the September 11 terrorist attacks could potentially apply to Iran based on the country's ties with Al Qaeda.⁸¹ However, in a June 28, 2019, letter to House Foreign Affairs Committee Chairman Eliot Engel, Assistant Secretary of State for Legislative Affairs Mary Elizabeth Taylor stated that "the Administration has not, to date, interpreted either [the 2001 or 2002] AUMF as authorizing military force against Iran, except as may be necessary to defend U.S. or partner forces engaged in counterterrorism operations or operations to establish a stable, democratic Iraq."

The killing of IRGC-QF Commander Soleimani in a U.S. drone strike in Baghdad in January 2020 dramatically increased congressional attention to U.S.-Iran tensions and specifically to the authority under which Soleimani was killed and whether that authority might be used to justify further military action. Immediately after the strike, House Speaker Nancy Pelosi said in a statement that the Administration launched the strike that killed Soleimani "without an Authorization for Use of Military Force (AUMF) against Iran" and "without the consultation of the Congress," and called for Congress to be "immediately briefed on this serious situation."⁸²

Two days later, on January 4, 2020, President Trump submitted a notification to the Speaker of the House and President Pro Tempore of the Senate of the Soleimani drone strike, as pursuant to the War Powers Resolution (P.L. 93-148), including the constitutional and legislative authority for the action. However, according to a media report, the notification "only contained classified information, according to a senior congressional aide, likely detailing the intelligence that led to the action."⁸³ Speaker Nancy Pelosi criticized the decision to classify the notification in its entirety as "highly unusual."⁸⁴ In statements after the strike, National Security Adviser Robert O'Brien asserted that the Authorization for Use of Military Force Against Iraq Resolution of 2002 ("2002 AUMF"; P.L. 107-243) provided the President authority to direct the strike against General Soleimani in Iraq.⁸⁵ The House voted to repeal the 2002 AUMF on January 30, 2020, when it passed the No War Against Iran Act (H.R. 550); no action has been taken by the Senate.

In response to the strike, numerous pieces of legislation were introduced both commending and condemning the Administration for the action (for more, see CRS Report R46148, U.S. Killing of Qasem Soleimani: Frequently Asked Questions). Perhaps most significant were two resolutions that would direct the President to terminate the involvement of U.S. forces in conflict with Iran.

H.Con.Res. 83, introduced by Representative Elissa Slotkin on January 8, 2020, pursuant to Section 5(c) of the War Powers Resolution. The resolution would direct the President "to terminate the use of United States Armed Forces to engage in hostilities in or against Iran or any part of its government or military," unless Congress specifically authorizes such use of the armed forces, or if such force is necessary and appropriate to defend the United States or its armed forces against "imminent attack." The House voted to adopt H.Con.Res. 83 by a 224-194 vote on January 9, 2020; no action has been taken by the Senate. Questions have been raised about the constitutionality and effect of Section 5(c) concurrent resolutions.

S.J.Res. 68, introduced by Senator Tim Kaine on January 9, 2020, pursuant to Section 1013 of the Department of State Authorization Act, Fiscal Years 1984 and 1985 (50 U.S.C. § 1546a). The resolution would have directed the President to "terminate the use of U.S. armed forces for hostilities" with Iran (changed from an earlier version that would have required "removal" of U.S. armed forces, perhaps a reflection of concern that the original language might precipitate changes in current deployments). The Senate voted to adopt the resolution by a 55-45 vote on February 13, and the House passed it by a 227-186 vote on March 11. President Trump vetoed the resolution on May 6, 2020, describing it as an "insulting" election ploy by congressional Democrats.⁸⁶ The statement also stated that the resolution's implication that "the President's constitutional authority to use military force is limited to defense of the United States and its forces against imminent attack" was "incorrect." The Senate failed to override the veto by a vote of 49-44 on May 7, 2020.

Possible Issues for Congress

Given ongoing tensions with Iran, Members are likely to continue to assess and perhaps try to shape the congressional role in any decisions regarding whether to commit U.S. forces to potential hostilities. In assessing its authorities in this context, Congress might consider, among other things, the following:

• Does the President require prior authorization from Congress before initiating hostilities with Iran? If so, what actions, under what circumstances, ought to be covered by such an authorization? If not, what existing authorities provide for the President to initiate hostilities?
• If the executive branch were to initiate and then sustain hostilities against Iran without congressional authorization, what are the implications for the preservation of Congress's role, relative to that of the executive branch, in the war powers function? How, in turn, might the disposition of the war powers issue in connection with the situation with Iran affect the broader question of Congress's status as an equal branch of government, including the preservation and use of other congressional powers and prerogatives?
• The Iranian government may continue to take aggressive action short of directly threatening the United States and its territories while it continues policies opposed by the United States. What might be the international legal ramifications for undertaking a retaliatory, preventive, or preemptive strikes against Iran in response to such actions without a U.N. Security Council mandate?

Conflict with, or increased military activity in or around, Iran could generate significant costs, financial and otherwise. With that in mind, Congress could consider the following:

• The potential costs of heightened U.S. operations in the CENTCOM area of operations, particularly if they lead to full-scale war and significant postconflict operations.
• The need for the United States to reconstitute its forces and capabilities, particularly in the aftermath of a major conflict.
• The impact of the costs of war and post conflict reconstruction on U.S. deficits and government spending.
• The costs of persistent military confrontation and/or a conflict in the Gulf region to the global economy.
• The extent to which regional allies, and the international community more broadly, might contribute forces or resources to a military campaign or its aftermath.

Figure 2. Iran, the Persian Gulf, and the Region

Sources: Created by CRS using data from the U.S. Department of State, ESRI, and GADM.

Figure 3. Shipping Lanes in the Strait of Hormuz and Persian Gulf

Source: CRS. Based on, and includes, map by Navy of the United Kingdom.

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