U.S.-Iran Tensions and Implications for U.S. Policy

Updated July 29, 2019 (R45795)
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Since May 2019, U.S.-Iran tensions have escalated. The Trump Administration, following its 2018 withdrawal from the 2015 multilateral nuclear agreement with Iran (Joint Comprehensive Plan of Action, JCPOA), has taken several steps in its campaign of applying "maximum pressure" on Iran. Iran and Iran-linked forces have targeted commercial ships and infrastructure in U.S. partner countries. U.S. officials have stated that Iran-linked threats to U.S. forces and interests, and attacks on several commercial ships in May and June 2019, have prompted the Administration to send additional military assets to the region to deter future Iranian actions. However, Iran's downing of a U.S. unmanned aerial aircraft might indicate that Iran has not been deterred, to date.

President Donald Trump has said he prefers a diplomatic solution over moving toward military confrontation, including a revised JCPOA that encompasses not only nuclear issues but also broader U.S. concerns about Iran's support for regional armed factions. During May-June 2019, the Administration has placed further pressure on Iran's economy. By expanding U.S. sanctions against Iran, including sanctioning its mineral and petrochemical exports, and Supreme Leader Ali Khamene'i. Iranian leaders have refused to talk directly with the Administration, and Iran has begun to exceed some nuclear limitations stipulated in the JCPOA. High-ranking officials from several countries, including Japan, Germany, France, Oman, Qatar, and Iraq, have visited Tehran to try to de-escalate U.S.-Iran tensions.

An expanding action-reaction dynamic between the United States and Iran has the potential to escalate into significant conflict. The United States military has the capability to undertake a large range of options against Iran in the event of conflict, both against Iran directly and against its regional allies and proxies. However, Iran's alliances with and armed support for armed factions throughout the region, and its 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.

Members of Congress have received additional information from the Administration about the causes of the uptick in U.S.-Iran tensions and Administration planning for further U.S. 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 Recent U.S.-Iran Tensions

U.S.-Iran relations have been mostly adversarial—but with varying degrees of intensity—since the 1979 Islamic Revolution in Iran. Since then, U.S. officials consistently have identified Iran's support for militant Middle East groups as 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 block Iran's ability to purchase advanced conventional weaponry and to develop ballistic missiles.

In May 2018, the Trump Administration withdrew the United States from the 2015 nuclear agreement (Joint Comprehensive Plan of Action, JCPOA) on the grounds that the agreement did not address a broad range of U.S. concerns about Iranian behavior and would not permanently preclude the potential for Iran to develop a nuclear weapon.1 Administration officials, such as Secretary of State Michael Pompeo and his senior adviser on Iran affairs, Ambassador Brian Hook, say that Administration policy is to apply "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 statements also suggest that an element of the policy could be to create enough economic difficulties to stoke unrest in Iran, possibly to the point where the regime collapses.3

As the Administration has pursued its policy of maximum pressure, bilateral tensions have escalated significantly, with U.S. steps going beyond the reimposition of all U.S. sanctions that were in force before JCPOA went into effect in early 2016. Key developments since April 2019 include the following:

Iranian Responses and Actions

Iran has responded to the additional U.S. sanctions in part by demonstrating its ability to harm global commerce and other U.S. interests and to raise new concerns about Iran's nuclear activities. Iran also could be trying to cause international actors, such as Russia, European countries, and countries in Asia that depend on stable oil supplies, to put pressure on the Trump Administration to reduce its sanctions pressure on Iran.

Attacks on Tankers

Actions by Iran's Regional Allies

In addition to direct Iranian action, Iran's allies in the region conducted attacks that might have been linked to the escalating U.S.-Iran tension, although the groups in question did not announce direct linkage.

Iran and U.S. Downings 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.21 IRGC commander-in-chief Maj. Gen. Hossein Salami stated "The downing of the American drone is an open, clear and categorical message, which is: the defenders of the borders of Iran will decisively deal with any foreign aggression.… This is the way the Iranian nation deals with its enemies."

On June 20, 2019, 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 shootdown.22 The United States did reportedly launch a cyberattack against Iranian radar and missile batteries and an Iranian spy group that has ties to the Islamic Revolutionary Guard Corps (IRGC) and was connected to the tanker attacks that occurred one week earlier.23

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. Iran denied that any of its drones were shot down.

UK-Iran Tensions: Tanker Seizures

In July, an effort by the United Kingdom (UK) to enforce EU sanctions against Syria opened up a dispute between Iran and the UK that added to the tensions in the Gulf. On July 4, authorities from the British Overseas Territory Gibraltar, backed by British marines, impounded an Iranian tanker, the Grace I, off the coast of Gibraltar on the grounds that it was allegedly violating an EU embargo on the provision of oil to Syria. Iranian officials termed the seizure an illegitimate 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 escorting the tanker. 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, 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 there. Secretary Hunt stated that the UK had "made clear in public that [it] would be content with the release of Grace I if there were sufficient guarantees the oil would not go to any entities sanctioned by the EU."24

President Donald Trump and other senior U.S. officials publicly supported the UK position, but U.S. officials did not indicate that the United States would take any specific new action to retaliate for the Iranian seizure of the Stena Impero. Secretary of State Pompeo has said that "the responsibility…falls to the United Kingdom to take care of their ships."25 At the same time, UK officials stated that they remained committed to the JCPOA and would not join the Trump Administration campaign of maximum pressure on Iran.

Parallels to Past Incidents in the Gulf26

Iran's apparent attacks on tankers in May and June share some characteristics with events in the mid-to-late 1980s during the 1980-88 Iran-Iraq war. 1987-88 represented the height of the so-called "tanker war," in which both Iran and Iraq were attacking ships in the Gulf. The United States backed Iraq during that war, and sought to limit and deter Iranian attacks on shipping, but there were several U.S.-Iran skirmishes in the Gulf. To protect commercial shipping, the United States launched "Operation Earnest Will" in July 1987, in which the United States reflagged 11 of Kuwait's oil tankers and the U.S. Navy escorted them through the Gulf. Almost immediately after the operation began, one of the tankers, the Bridgeton, was damaged by a large contact mine laid by Iran. In August 1987, U.S. forces captured the Iran Ajr, an Iranian landing craft being used for covert minelaying. However, Iran continued attacking, including with missiles; on October 16, 1987 an Iranian Silkworm missile struck on a U.S.-flagged Kuwaiti tanker, Sea Isle City, ten miles off Kuwait's Al Ahmadi port. In response to that attack, U.S. destroyers and Special Operations forces blew up an Iranian oil platform east of Bahrain. On April 14, 1988, an Iranian-laid mine struck the U.S. frigate Samuel B. Roberts on patrol in the central Gulf, an attack that led to an April 16, 1988 naval confrontation in which the United States, in Operation Praying Mantis, put a large part of Iran's naval force out of action, including sinking one of Iran's two frigates and rendering the other inoperable. On July 3, 1988, mistaking it for an attacking Iranian aircraft, the guided missile cruiser U.S.S. Vincennes shot down Iran Air commercial passenger flight 655, killing all aboard.

International Responses to the Current Dynamic

Responses by U.S. partners and other actors to the U.S.-Iran tensions appeared consistent with positions of major international players on the JCPOA. Secretary Pompeo's statement on June 13, 2019, "call[ed] upon all nations threatened by Iran's provocative acts to join us in that endeavor [of compelling Iran to return to the negotiating table]." However, key U.S. allies in Europe—all of whom criticized the U.S. withdrawal from the JCPOA—have not generally supported the U.S. position on the tensions thus far, tending instead to call for an easing of tensions and for "maximum restraint" by the United States.27 After the initial escalation of tensions in early May, Secretary of State Pompeo attended meetings with EU officials on May 13 to brief them on U.S. intelligence about the heightened Iranian threat. At the conclusion of the meetings, UK Foreign Secretary Jeremy Hunt stated "We [EU] are very worried about the risk of a conflict happening by accident, with an escalation unintended really on either side."28

As an example of the apparent EU hesitancy to back the U.S. maximum pressure campaign, several EU countries, including Germany, openly questioned U.S. assertions of Iranian responsibility for the June 13 attacks. The UK was an exception; its Foreign Office issued a statement saying: "It is almost certain that a branch of the Iranian military—the Islamic Revolutionary Guard Corps—attacked the two tankers on 13 June. No other state or non-state actor could plausibly have been responsible."29 After the United States released videos purporting to show the IRGC at one of the tankers, German Chancellor Angela Merkel said that there was "strong evidence" Iran committed the attacks, but the EU and the German foreign ministers maintained that they had not reached any conclusions and were continued to evaluate the evidence.30

Russia is a signatory to the JCPOA and a partner of Iran in Syria and on other issues, and Russia's deputy foreign minister warned the United States against using the latest tanker attacks to "further aggravate the situation in an anti-Iran sense."31 On the other hand, Saudi Arabia, a major adversary of Iran, called for "decisive" action to protect global energy supplies that it said are threatened by Iran.32

JCPOA-Related Iranian Responses33

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 the remaining parties' providing the agreement's benefits. 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.34

Tehran also threatened to reconstitute and resume the country's pre-JCPOA nuclear activities. According to Iranian officials, the country can rapidly reconstitute its fissile material production capability and has begun preparations for expanding its uranium enrichment program since the May 2018 U.S. announcement described above.35

Several meetings of the JCPOA-established Joint Commission since the U.S. withdrawal have not produced a firm Iranian commitment to the agreement.36 Tehran has argued that the remaining JCPOA participants' efforts have been inadequate to sustain the agreement's benefits for Iran. In May 8 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.

Behrouz Kamalvandi, spokesperson for the Atomic Energy Organization of Iran, warned on June 17, 2019, that Iran would exceed on June 27 the JCPOA-mandated quantitative limit on Iran's LEU stockpile if the agreement's other participants did not meet Tehran's demands. The Iranian government stated that it would resume full compliance with the JCPOA if the remaining participants agree during a 60-day period following the May 8, 2019, announcement to meet Tehran's demands (by July 7). However, Kamalvandi and other Iranian officials warned that, absent such an agreement, Iran would cease to accept any constraints on the concentration of Iranian-produced LEU. According to two July reports from IAEA Director General Amano, both the quantity of Iran's LEU stockpile, as well as the that LEU's concentration of the relevant fissile isotope uranium-235, currently exceed JCPOA-mandated limits.37 Whether this action constitutes noncompliance with the JCPOA is unclear.38

Iran has previously threatened to resume work on a nuclear reactor according to its original design. Tehran has rendered that reactor's original core inoperable pursuant to the JCPOA, which also commits Tehran to redesign and rebuild the reactor based on a design agreed to by the P5+1.

U.S. Military Deployments

In response to the escalating tensions with Iran, the United States has added forces and military capabilities in the region, beyond the accelerated deployment of the U.S.S. Abraham Lincoln and associated forces, discussed above. The deployments have added several thousand U.S. military personnel 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.39 Defense Department officials have indicated that the additional deployments since early May restore forces who were redeployed from the region a few years ago, and that the new deployments do not represent a buildup in preparation for any U.S. offensive against Iran.40

Scenarios and Possible Outcomes

Events could take any of several directions that might affect congressional oversight and authorization or limitations on the U.S. use of military force, administration and congressional steps to support regional partners potentially affected by conflict, or new sanctions measures.

Further Escalation

U.S. and Iranian officials have said they do not want armed conflict. However, leaders on each side have said they will respond with force if the other attacks, increasing the potential for conflict.

The Iranian leadership insists that U.S. sanctions be eased, and, in order to pressure on the United States to do so, Iran could undertake further actions against U.S. personnel or interests, including taking further actions against international shipping in the Gulf. The IRGC's Qods Force (IRGC-QF) arms, trains, and advises allies and proxies throughout the Middle East region, including in Syria, Lebanon,44Iraq, Yemen, Bahrain, and Afghanistan.45The IRGC-QF has supplied these regional allies with rockets, short-range ballistic missiles, and other weaponry with which they could attack commercial or naval ships, bases, civilian targets, U.S. or other military and civilian personnel in the region, and any number of other targets.46The annual State Department report on international terrorism has consistently asserted that Iran and its key ally, Lebanese Hezbollah, have a vast network of agents in Europe, Latin America, and elsewhere that could act against U.S. personnel and interests outside the Middle East.47

Status Quo

It is possible that the U.S.-Iran tensions could remain, but neither escalate nor de-escalate. The tensions might not evolve to military conflict, but might not result in talks that lead to a potential resolution of the U.S.-Iran differences.


Either the United States or Iran could try to de-escalate the tensions. Iran, for its part, could potentially take up U.S. offers to negotiate a broader, revised JCPOA, although U.S. demands for a new JCPOA are extensive and Iranian leaders have categorically rejected them. Iran might also offer to de-escalate by pledging to refrain from any interference with international shipping or by reducing its naval operations in the Gulf. The EU or other actors could also potentially produce a de-escalation by formulating policies that provide Iran with the economic benefits of the JCPOA.

President Trump and other senior officials have stated several times since May that the United States wants to de-escalate tensions, avoid conflict with Iran, and negotiate a revised JCPOA.48 On June 2, Secretary Pompeo stated that: "We are prepared to engage in a conversation [with Iran] with no preconditions, we are ready to sit down." Before departing on a trip to the region to discuss the Iran issue, Secretary Pompeo stated on June 23: "The President has said repeatedly we want a brighter future for the people of the Islamic Republic of Iran. Those negotiations are the gateway. That's how we'll ultimately achieve this."49 For Iran's part, Foreign Minister Zarif visited the United Nations in July 2019 and offered, in return for the United States' return to implementing its JCPOA commitments and lifting of U.S.JCPOA-related sanctions, to accelerate Iran's ratification of the Additional Protocol to its IAEA safeguards agreement ahead of the JCPOA-mandated schedule.50

Another question that arises is what are the channels or mechanisms to de-escalate tensions through direct or, more likely, indirect talks. The United States and Iran do not have diplomatic relations and there have been no known direct, high-level talks between Iran and the United States since the Trump Administration withdrew from the JCPOA. This absence of relations likely means that any U.S.-Iran de-escalation will need to be facilitated by mediators. Secretary Pompeo has discussed the escalating U.S.-Iran tensions in at least one direct contact with Sultan Qaboos of Oman, who in the past has mediated U.S.-Iran talks.51 Japan's Prime Minister Shinzo Abe sought to mediate a de-escalation during his visit to Iran on June 12-13, 2019, the first visit to Iran by a Japanese leader since the Islamic revolution. That visit followed one by Germany's foreign minister to Tehran in early June. Iranian leaders have said they would not engage in talks with the United States unless the United States resumes implementing the JCPOA. In early July, French President Macron spoke with President Rouhani and sent a top aide, Emmanuel Bonne, to Tehran for mediation talks. President Trump also confirmed on July 19, 2019 that he authorized Senator Rand Paul to engage in diplomatic discussions with Foreign Minister Zarif; Sen. Paul reportedly met with Zarif in New York in July.52

A possible means to de-escalate tensions with Iran is for the United States to relax its maximum pressure policy on Iran. Doing so could take the form of exercising waivers or exceptions for certain transactions with Iran. One example could be the restoration of the sanctions exceptions for the purchase of Iranian oil—a step that could remove Iran's objections to being largely denied the ability to export that vital commodity. The EU countries could take additional steps that might satisfy Iran's economic demands, including using a new EU trading mechanism (Instrument in Support of Trading Exchanges, INSTEX) to purchase Iranian oil or, potentially, providing Iran with loans or grants that could compensate Iran for the loss of its oil sales.

U.S. Military Action: Considerations, Options, and Risks

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."53 As such, the military could be used in a variety of ways to try and contain and dissuade Iran from prosecuting its "hostile and destabilizing actions." These ways range from 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 retaliatory Iranian action and/or the escalation of a crisis.

U.S. military action may not be the appropriate tool to achieve systemic change within the Iranian regime, and may in fact worsen the situation for Iranians sympathetic to a change of regime. Employing overt military force is likely to strengthen anti-American elements within the Iranian Government. 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 note that China and Russia represent the key strategic challenges to the United States today and into the future. As such, shifting 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.

U.S. officials have stated that the additional U.S. deployments since May are intended to "deter" Iran from taking any further provocative actions. Yet, the downing of the RQ-4A Global Hawk Unmanned Aerial Vehicle on June 20, 2019, demonstrates that deploying additional assets and capabilities has not necessarily succeeded in deterring Iran from using military force.

Still others contend that the risks of military inaction are greater than 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.54

For illustrative purposes only, below are some potential 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. And, the escalation of U.S.-Iran tensions has prompted Congress to assess its role in any decisions regarding whether to undertake military action against Iran, an issue that is discussed later in this report. The following discussion is based entirely on open-source materials.

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:

Congressional Responses

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

Some Members have expressed concern about the legal justification for military operations in or against Iran. On June 22, Senator Bernie Sanders (I-VT) cast doubt on the notion of a "limited strike," saying that "[attacking] another country with bombs … that's an act of warfare" and said that an attack on Iran would be, in his view, "unconstitutional."63 Although Presidents have long asserted wide-ranging authority to unilaterally initiate the use of military force, no legislation has been enacted authorizing the use of force against Iran, and several measures include provisions specifying that such authorization is not being granted.64 For instance, Section 9026 of Division C of H.R. 2740, as engrossed in the House on June 19, 2019, states that "Nothing in this Act may be construed as authorizing the use of force against Iran." H.R. 2500, the National Defense Authorization Act for FY2020, as reported in the House on June 19, 2019, contains a similar provision (Section 1225). On July 12, 2019, the House also passed, by a vote of 251-170, an amendment to H.R. 2500 that would prohibit funding for the use of force against Iran, with provisions clarifying that such a prohibition would not prevent the President from using necessary and appropriate force to defend U.S. allies and partners, consistent with the War Powers Resolution.

Other Members have positioned themselves as more generally supportive of broad discretion for the Administration to act. Senator Tom Cotton (R-AR) said on June 16th that "these unprovoked attacks on commercial shipping warrant a retaliatory military strike" and argued that the president had the authority to order such an attack.65 The day before, Senator Lindsey Graham (R-SC) made a similar argument, stating that "enough is enough" and called on President Trump to "be prepared to make Iran pay a heavy price by targeting their naval vessels and, if necessary, their oil refineries."66 On June 28, 2019, the Senate rejected by a 50-40 vote an amendment (S.Amdt. 883) to the FY2020 National Defense Authorization Act that would have prohibited the use of any funds to "conduct hostilities against the Government of Iran, against the Armed Forces of Iran, or in the territory of Iran, except pursuant to an Act or joint resolution of Congress specifically authorizing such hostilities."67

President Trump said in a June 24 interview that he believes he has the authority to direct strikes against Iran, and said that "I do like keeping them [Congress] abreast, but I don't have to do it, legally."68 On June 25, he tweeted that "any attack by Iran on anything American will be met with great and overwhelming force. In some areas, overwhelming will mean obliteration."69

At a Senate Foreign Relations Committee hearing on April 10, 2019, Secretary of State Pompeo, when asked if the Administration considers the use of force against Iran as authorized, answered that he would defer to Administration legal experts on that question. However, he suggested 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, asserting that "[Iran has] hosted Al Qaida. They have permitted Al Qaida to transit their country. [There's] no doubt there is a connection between the Islamic Republic of Iran and Al Qaida. Period. Full stop." Other analyses have characterized the relationship between Iran and Al Qaeda as "an on-again, off-again marriage of convenience pockmarked by bouts of bitter acrimony."70 Section 9025 of H.R. 2740 would repeal the 2001 AUMF within 240 days of enactment.71

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." In response, Chairmen Engel and Middle East Subcommittee Chairman Ted Deutch welcomed the Administration's apparent acknowledgment that "the 2001 and 2002 war authorizations do not apply to military action against Iran," but cautioned that "the Administration claims that the President could use these authorizations to attack Iran in defense of any third party he designates a partner."72 In reviewing the letter, two analysts have suggested additional related topics for potential congressional oversight, including which groups are carrying out such counterterrorism operations, where they are doing so, and what nations or groups threaten them.73

Additionally, some Members seeking to prevent the Administration from pursuing military action against Iran have introduced several standalone measures prohibiting the use of funds for such operations, such as the Prevention of Unconstitutional War with Iran Act of 2019 (H.R. 2354/S. 1039) which would prevent the use of any funds for "kinetic military operations in or against Iran" except in case of an imminent threat.

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:

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

Figure 1. Iran, the Persian Gulf, and the Region

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

Appendix A. Selected Statements by U.S. and Iranian Leaders on Recent Tensions74


U.S. Statements

Iranian Statements

April 22

Pompeo: "We have watched Iran have diminished power as a result of our campaign. Their capacity to wreak harm around the world is absolutely clearly diminished." 


April 24


FM Zarif: "It is not a crisis yet, but it is a dangerous situation. Accidents, plotted accidents are possible.… The plot is to push Iran into taking action. And then use that."

April 30


Rouhani: "America's decision that Iran's oil exports must reach zero is a wrong and mistaken decision, and we won't let this decision be executed and operational…In future months, the Americans themselves will see that we will continue our oil exports."

May 5

National Security Advisor John Bolton statement: "In response to a number of troubling and escalatory indications and warnings, the United States is deploying the USS Abraham Lincoln Carrier Strike Group and a bomber task force to the U.S. Central Command region to send a clear and unmistakable message to the Iranian regime that any attack on United States interests or on those of our allies will be met with unrelenting force. The United States is not seeking war with the Iranian regime, but we are fully prepared to respond to any attack, whether by proxy, the Islamic Revolutionary Guard Corps, or regular Iranian forces."


May 8


President Rouhani (via Twitter): "Starting today, Iran does not keep its enriched uranium and produced heavy water limited. The EU/E3+2 will face Iran's further actions if they can not fulfill their obligations within the next 60 days and secure Iran's interests. Win-Win conditions will be accepted."

May 9

President Trump: "I'd like to see [Iran] call me" to "make a deal, a fair deal"


May 12


Rouhani: "The pressures by enemies is a war unprecedented in the history of our Islamic revolution ... but I do not despair and have great hope for the future and believe that we can move past these difficult conditions provided that we are united."

May 14


Supreme Leader Khamenei: "There won't be any war. The Iranian nation has chosen the path of resistance"

May 15

President Trump (via Twitter): "I'm sure that Iran will want to talk soon."


May 19

President Trump (via Twitter): "If Iran wants to fight, that will be the official end of Iran. Never threaten the United States again!"


May 20

President Trump (via Twitter): "Iran will call us if and when they are ever ready. In the meantime their economy continues to collapse—very sad for the Iranian people!"

Rouhani: "Today's situation is not suitable for talks and our choice is resistance only."

May 27

President Trump: "I really believe that Iran would like to make a deal, and I think that's very smart of them, and I think that's a possibility to happen…It has a chance to be a great country with the same leadership,…We aren't looking for regime change—I just want to make that clear. We are looking for no nuclear weapons."


May 29

NSA Bolton: "I think it is clear these [tanker attacks] were naval mines almost certainly from Iran….There is no doubt in anybody's mind in Washington who was responsible for this."

Supreme Leader Khamenei (via Twitter): "We won't negotiate with Americans. Because there's no use negotiating and it's even harmful. Otherwise we have no problems negotiating with others & with Europeans."

June 2

Pompeo: "We are prepared to engage in conversation with no preconditions, we are ready to sit down" with Iran.


June 13

President Trump (via Twitter): "While I very much appreciate [Japanese Prime Minister] Abe going to Iran to meet with Ayatollah Ali Khamenei, I personally feel it is too soon to even think about making a deal. They are not ready, and neither are we!"

Pompeo: "Iran is lashing out because the regime wants our successful maximum pressure campaign lifted… Our policy remains an economic and diplomatic effort to bring Iran back to the negotiating table at the right time, to encourage a comprehensive deal that addresses the broad range of threats—threats today apparent for all the world to see—to peace and security."

Supreme Leader Khamenei (via Twitter): "We have no doubt in [PM Abe's] goodwill and seriousness; but regarding what you mentioned from U.S. president, I don't consider Trump as a person deserving to exchange messages with; I have no response for him & will not answer him."

June 17

President Trump, on alleged Iranian attacks in the Gulf: "So far, it's been very minor"


June 20

President Trump: "I find it hard to believe [Iran shooting down a U.S. drone] was intentional…I have a feeling that it was a mistake made by somebody that shouldn't have been doing what they did."


June 21

President Trump: "I'm not looking for war, and if there is, it'll be obliteration like you've never seen before."


June 22

President Trump: "We're not going to have Iran have a nuclear weapon. And when they agree to that, they are going to have a wealthy country, they're going to be so happy and I'm going to be their best friend."


Appendix B. Selected Actions by U.S. and Iran

May 5

National Security Advisor John Bolton releases a statement announcing deployment of USS Abraham Lincoln Carrier Strike Group and bomber task force to Gulf

May 12-13

Four tankers anchored off the UAE are damaged in attacks in the Gulf; U.S. initial assessment attributes the attacks to Iran

May 24

Department of Defense officials announce deployment of 1,500 troops to region (900 new deployments and 600 as part of Patriot battalion deployment extension)

May 29

NSA Bolton says Iran "almost certainly" responsible for May tanker attacks; Iran rejects charges

June 13

Two Saudi tankers attacked in the Gulf of Oman; Secretary of State Mike Pompeo says intelligence indicates Iranian responsibility, a claim Iran denies

June 14

Iran reportedly shoots at U.S. MQ-9 Reaper drone monitoring tankers

June 18

Then-Acting Secretary of Defense Shanahan announces deployment of 1,000 troops to Middle East for "defensive purposes"

June 20

Iran shoots down a U.S. RQ-4A Global Hawk unmanned aerial surveillance aircraft, claiming it violated Iranian airspace; U.S. officials state it was in international airspace.

In response, President Trump reportedly orders retaliatory strikes on Iranian radar installations and other targets, but then cancels that order due to concerns that the attack would be disproportionate to the downing of the drone

President Trump reportedly approves cyberattack on Iranian rocket and missile launch systems

July 4

British navy impounds an Iranian tanker off the coast of Gibraltar; tanker is suspected of violating EU oil embargo on Syria, which Iran denies

July 10

A British warship in the Gulf blocks an attempt by Iranian vessels to interdict a British tanker, according to U.S. officials

Chairman of the Joint Chiefs of Staff General Joseph Dunford states that the U.S. will seek to build a maritime coalition

July 18

President Trump announces the downing of an Iranian drone via electronic jamming in "defensive action" over the Strait of Hormuz; Iran denies any of its drones were shot down

U.S. defense officials say 500 additional U.S. troops to Saudi Arabia

July 19

Iran announces seizure of British-flagged tanker near the Strait of Hormuz, claiming variously that it violated Iranian waters, was polluting the Gulf, collided with an Iranian vessel, or that the seizure was retribution for Great Britain's July 4 seizure of an Iranian tanker near Gibraltar

Figure B-1. Shipping Lanes in the Strait of Hormuz and Persian Gulf

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

Author Contact Information

Kenneth Katzman, Specialist in Middle Eastern Affairs ([email address scrubbed], [phone number scrubbed])
Kathleen J. McInnis, Specialist in International Security ([email address scrubbed], [phone number scrubbed])
Clayton Thomas, Analyst in Middle Eastern Affairs ([email address scrubbed], [phone number scrubbed])



For information on the JCPOA and the rationale for the U.S. withdrawal, see CRS Report R43333, Iran Nuclear Agreement and U.S. Exit, by Paul K. Kerr and Kenneth Katzman.


Speech by Secretary of State Michael Pompeo, Heritage Foundation, May 21, 2018; Testimony of Ambassador Brian Hook before the House Foreign Affairs Subcommittee on Middle East, North Africa, Hearing on U.S.-Iran Relations. June 19, 2019.


Speech by Secretary of State Pompeo, Heritage Foundation, op. cit.


Statement from the President on the Designation of the Islamic Revolutionary Guard Corps as a Foreign Terrorist Organization, April 8, 2019, at https://www.whitehouse.gov/briefings-statements/statement-president-designation-islamic-revolutionary-guard-corps-foreign-terrorist-organization/.


Department of State, Office of the Spokesperson, Factsheet: Designation of the Islamic Revolutionary Guard Corps, April 8, 2019.


State Department Factsheet, April 22, 2019, at https://www.state.gov/advancing-the-u-s-maximum-pressure-campaign-on-iran/.


Letter from Mary Elizabeth Taylor, Assistant Secretary of State for Legislative Affairs, to Senator James Risch, Chairman of the Senate Foreign Relations Committee. May 3, 2019.


The text of the announcement can be found at https://www.whitehouse.gov/briefings-statements/statement-national-security-advisor-ambassador-john-bolton-2/.


The text of the Order can be found at https://www.whitehouse.gov/presidential-actions/executive-order-imposing-sanctions-respect-iron-steel-aluminum-copper-sectors-iran/.


Letter from Secretary of State Michael R. Pompeo to Senate Foreign Relations Committee Chairman James E. Risch, May 24, 2019.


Department of Defense Briefing on Iran, May 24, 2019. For analysis on Saudi Arabia, see CRS Report RL33533, Saudi Arabia: Background and U.S. Relations, by Christopher M. Blanchard.


Pamela Falk, "Oil tanker attack probe reveals new photos, blames likely 'state actor,'", CBS News, June 7, 2019.


Statement by the Secretary of State, June 13, 2019.


For analysis on Iraq, see CRS Report R45025, Iraq: Background and U.S. Policy, by Christopher M. Blanchard.


Department of Defense Briefing on Iran. May 24, 2019, op. cit.


"Rockets strike near U.S.-linked oil installation and military bases in Iraq; 3 wounded," Washington Post, June 19, 2019.


"U.S. says Saudi pipeline attacks originated in Iraq: Wall Street Journal," Reuters, June 28, 2019. For analysis on the Yemen conflict, see CRS Report R43960, Yemen: Civil War and Regional Intervention, by Jeremy M. Sharp.


Sadursan Raghavan, "Yemeni rebels claim new drone attack on Saudi airport," Washington Post, June 17, 2019.




"The Taliban Claimed an Attack on U.S. Forces. Pompeo Blamed Iran," Washington Post, June 16, 2019.


See U.S. Central Command Statement at https://www.centcom.mil/MEDIA/STATEMENTS/Statements-View/Article/1881682/us-central-command-statement-iranians-shoot-down-us-drone/.


President Donald Trump interview on "Meet the Press," June 23, 2019.


"U.S. Seeks Other Ways to Stop Iran Shy of War," New York Times, June 24, 2019.


UK Foreign and Commonwealth Office, Statement by Jeremy Hunt to Parliament, July 22, 2019.


"In naval confrontation with Iran, great Britain can find neither ships nor friends," Defense News, July 25, 2019.


Much of this textbox is derived from Ronald O'Rourke, U.S. Naval Institute Proceedings, "The Tanker War," May 1988; and CRS Issue Brief IB87145, "Persian Gulf: U.S. Military Operations," January 19, 1989.


"Distrustful of Both Sides, Europe Urges 'Maximum Restraint" as Tensions Escalate," New York Times, June 15, 2019.


"Pompeo crashes Brussels meeting of E.U. diplomats but changes few minds on Iran," Washington Post, May 13, 2019.


"UK joins US in accusing Iran of tanker attacks as crew held," The Guardian, June 14, 2019.


"Germany says there is 'strong evidence' Iran behind tanker attacks," Deutsche Welle, June 18, 2019.


"Russia Warns Against Using Tanker Attacks to Pressure Iran," Reuters, June 13, 2019.


"Saudi Arabia calls for 'decisive' action over tanker attacks," Al Jazeera, June 15, 2019.


This section was prepared by Paul K. Kerr. Specialist in Nonproliferation. For additional details, see CRS Report RL34544, Iran's Nuclear Program: Status, by Paul K. Kerr.


"Letter of Foreign Minister to UN Secretary General," May 10, 2018.


"Iran Can Resume 20 Per Cent Uranium Enrichment Only in 5 Days: Salehi," Iranian Students News Agency (ISNA), August 22, 2017; "AEOI Chief: Iran Able to Resume 20% Enrichment in Maximum 5 Days," FARS News Agency, August 22, 2017.


Unless otherwise noted, this paragraph is based on "AEOI Chief: Iran No More Remaining Restricted To 300 Kg Ceiling For Enriched Uranium," Fars News Agency, May 8, 2019; "Iran: Report Publishes Text of Supreme National Security Council Statement on Nuclear Deal," Tehran Fararu Online, May 8, 2019; Revision: Iran: President Ruhani Heralds New Nuclear Deal Strategy, Announces 60-Day Deadline, May 8, 2019; Statement from Supreme National Security Council of the Islamic Republic of Iran, May 8, 2019; "Spokesman: Iran to Cross 300kg Uranium Stockpile Borderline in 10 Days," FARS News Agency, June 17, 2019.


Verification and Monitoring in the Islamic Republic of Iran in Light of United Nations Security Council Resolution 2231 (2015), Report by the Director General, GOV/INF/2019/8. July 1, 2019; Verification and Monitoring in the Islamic Republic of Iran in Light of United Nations Security Council Resolution 2231 (2015), Report by the Director General, GOV/INF/2019/9, July 8, 2019.


For additional details, see CRS Report R40094, Iran's Nuclear Program: Tehran's Compliance with International Obligations, by Paul K. Kerr.


Paul Sonne and Missy Ryan, "Amid tensions with Iran, White House mulls U.S. military request to send more forces to the Middle East," Washington Post, May 23, 2019.


Gordon Lubold and Michael Gordon, "U.S. Deploys Forces to Mideast to Mideast to Deter Iran," Wall Street Journal, May 5, 2019.


Department of Defense Briefing on Iran, May 24, 2019, op. cit.


Doha Madani, "U.S. Sending 1,000 troops to Middle East amid heightened tension with Iran," NBC, June 18, 2019.


Robert Burns, "Officials: US putting troops back in Saudi Arabia," Associated Press, July 21, 2019.


For analysis on Syria and on Lebanon see CRS Report RL33487, Armed Conflict in Syria: Overview and U.S. Response, coordinated by Carla E. Humud and CRS Report R44759, Lebanon, by Carla E. Humud.


"Iran's Playbook—Deconstructing Tehran's Regional Strategy," Soufan Center, May 2019, at https://thesoufancenter.org/irans-playbook-deconstructing-tehrans-regional-strategy-by-the-soufan-center/.


Ambassador Brian Hook, "The Iranian Regime's Transfer of Arms to Proxy Groups," November 29, 2018.


Department of State. Country Reports on Terrorism: 2017.


Alyza Sebenius, "Trump Says 'Always a Chance' of War With Iran But Prefers Talks," Bloomberg, June 5, 2019.


Secretary of State Michael R. Pompeo Remarks to Press, Joint Base Andrews, June 23, 2019.


"Iran's Foreign Minister Proposes Modest Deal to End Impasse With U.S," New York Times, July 18, 2019.


Department of State, Secretary Pompeo's Call with Omani Sultan Qaboos bin Sa'id Al Sa'id. May 16, 2019.


"Trump: Sen. Rand Paul to help with Iran negotiations," Politico, July 19, 2019.


The White House, A Look at the U.S. Strategy for Iran, February 13, 2019, at https://www.whitehouse.gov/articles/look-u-s-strategy-iran/.


Kathleen J. McInnis (2005) Extended deterrence: The U.S. credibility gap in the Middle East, The Washington Quarterly, 28:3, 169-186, DOI: 10.1162/0163660054026489.


For more information on the respective U.S. and UK protection operations see: "U.S., Europe Have Different Plans to Patrol Persian Gulf." Washington Post, July 25, 2019. For information on the protection operations of the 1980s, see: CRS Report IB87145. Persian Gulf: U.S. Military Operations, by Ronald O'Rourke. http://cdbapps/CRSx/productdetails/147092; and U.S. Naval Institute Proceedings. The Tanker War, by Ronald O'Rourke. https://www.usni.org/magazines/proceedings/1988/may/tanker-war.


Edward Wong, "Trump Imposes New Sanctions on Iran, Adding to Tensions," New York Times, June 24, 2019.


Bradley Peniston, "Operation Earnest Will," at http://www.navybook.com/no-higher-honor/timeline/operation-earnest-will/.


"Trump Administration Prepares Multiple Military Options for Iran, Including Airstrikes and Setting Up Ground Invasion." Newsweek, May 14, 2019.


Emanuele Ottolenghi, "Only Crippling Sanctions Will Stop Iran," New York Times, March 2, 2012.


Eric Schmitt and Julian Barnes, "White House Reviews Military Plans against Iran, in Echoes of Iraq War," New York Times, May 13, 2019.


Adam Taylor, "A conflict with Iran would not be like the Iraq War. It would be worse," Washington Post, May 14, 2019.


IISS, The Military Balance: 2019, Iran.


Grace Segers, "Bernie Sanders says Trump 'helped create the crisis' in Iran," CBS News, June 22, 2019.


Some analysts have suggested that the 1973 War Powers Resolution (P.L. 93-148), which requires the President to notify Congress when U.S. armed forces are introduced into hostilities or situations of imminent hostilities and withdraw those forces within 60 to 90 days unless Congress authorizes such action, might also represent a check on the President's authority under Article II of the Constitution. Scott Anderson, "When Does the President Think He Can Go To War With Iran?" Lawfare, June 24, 2019. For more, see CRS Report R42699, The War Powers Resolution: Concepts and Practice, by Matthew C. Weed.


Nicholas Wu, "Trump says he doesn't want war, but Sen. Cotton called for 'retaliatory strikes' on Iran," USA Today, June 16, 2019.


Lindsey Graham, Twitter, June 15, 2019. https://twitter.com/LindseyGrahamSC/status/1140063250438078466.


For more on arguments around congressional attempts to limit military action via appropriations, see CRS Report R41989, Congressional Authority to Limit Military Operations, by Jennifer K. Elsea, Michael John Garcia, and Thomas J. Nicola.


Saagar Enjeti and Jordan Fabian, "EXCLUSIVE: Trump: I do not need congressional approval to strike Iran," The Hill, June 24, 2019.


President Trump (@realDonaldTrump), Twitter, June 25, 2019, 7:42 AM.


Ned Price, "Why Mike Pompeo Released More bin Laden Files," Atlantic, November 8, 2017. See also Barbara Slavin, "Expediency and betrayal: Iran's relationship with al-Qaeda," Al-Monitor Iran Pulse, September 7, 2018.


For background, see CRS Report R43983, 2001 Authorization for Use of Military Force: Issues Concerning Its Continued Application, by Matthew C. Weed.


Engel & Deutch Receive State Department Answer on Old War Authorizations & Iran, Committee on Foreign Affairs Press Release, June 28, 2019. While the Trump Administration has previously stated that it views the 2001 AUMF as authorizing U.S. action in defense of partner forces participating in counter-Islamic State operations in Syria, one analyst argues that the State Department's letter represents "the first time the executive branch has publicly extended this collective self-defense theory to the 2002 AUMF." Scott R. Anderson, "Parsing the State Department's Letter on the Use of Force Against Iran," Lawfare, July 3, 2019.


Tess Bridgeman and Ryan Goodman, "Unpacking the State Dept Acknowledgment that 2001 and 2002 AUMFs Don't Authorize War Against Iran," Just Security, July 3, 2019.


The quotes in this table are from various public sources available to congressional clients from the authors upon request.