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August 8, 2024

U.S. Extended Deterrence and Regional Nuclear Capabilities

One of the stated goals of U.S. nuclear weapons policy is to extend deterrence to over 30 U.S. “allies and partners” and assure these countries that the United States will come to their aid, including potentially by using U.S. nuclear weapons, if they are attacked. The 2010, 2018, and 2022 Nuclear Posture Reviews (NPRs), which are periodic assessments of U.S. nuclear policy, argued for strengthening extended deterrence and posited that such deterrence supported U.S. nuclear nonproliferation goals. The 2022 NPR stated:

Allies must be confident that the United States is willing and able to deter the range of strategic threats they face, and mitigate the risks they will assume in a crisis or conflict.… Extended nuclear deterrence contributes to U.S. non-proliferation goals by giving Allies and partners confidence that they can resist strategic threats and remain secure without acquiring nuclear weapons of their own.

Congress authorizes and appropriates funds for, and conducts oversight of, U.S. deterrence policies and their implementation, as well as U.S. defense and other cooperation with allies and partners.

Evolution of Extended Deterrence

Since the beginning of the Cold War, as part of its participation in NATO’s collective defense commitment enshrined in the 1949 North Atlantic Treaty, the United States threatened to use military force, including the first use of U.S. nuclear weapons, in response to an act of Soviet armed aggression against Western Europe. Such limited U.S. nuclear use carried the possibility of escalation to an all-out nuclear war between the United States and the Soviet Union; U.S. policymakers deemed this a credible deterrent of potential Soviet attack. The United States also eventually provided assurances to several allies in Asia that it would back their security with U.S. nuclear weapons.

Since the late 1950s, successive U.S. Administrations have expressed concerns about the security threats posed by the possible proliferation of nuclear weapons. During the negotiations on the Treaty on the Non-Proliferation of Nuclear Weapons in the 1960s, several U.S. allies sought additional U.S. security assurances while pledging that they would not develop nuclear weapons. U.S. policy has thus sought to extend the U.S. “nuclear umbrella” over allies in Europe and Asia in part as a means to reduce the incentive for them to acquire their own nuclear weapons.

Policymakers and experts in allied countries may debate the extent to which U.S. official statements, capabilities, and plans to defend them constitute a credible security commitment. If an allied government doubts the U.S. extended deterrence commitment, the government may respond by requesting additional U.S. security assurances

or by improving its own perceived security through other means. These means may include the development of independent or cooperative nuclear weapons capabilities.

After the end of the Cold War, the United States reduced its nuclear forces, including those stationed abroad, and narrowed the range of contingencies for which the United States would consider the first use of nuclear weapons. The 2010 and 2022 NPRs, which articulated a reduced role for nuclear weapons in U.S. military planning, emphasized the importance for extended deterrence of U.S. and allied nonnuclear capabilities, such as theater missile defense, and the forward presence of U.S. conventional forces. Allied perceptions of U.S. security commitments may be sensitive to U.S. domestic debates about possible shifts in U.S. nuclear policy or posture.

U.S. Regional Nuclear Capabilities

During the Cold War, the United States deployed various nonstrategic nuclear weapons, including capabilities that later were eliminated as part of U.S.-Soviet arms control and unilateral commitments, to allied countries. Today, the United States extends deterrence to its allies through forward-deployed nuclear weapons, U.S.-based aircraft capable of conventional or nuclear missions that could be deployed in a crisis, and strategic nuclear forces. (See CRS In Focus IF10519, Defense Primer: Strategic Nuclear Forces.)

The primary stated purpose of U.S. regional nuclear deterrence capabilities is to deter the limited use of nuclear weapons by Russia, the People’s Republic of China (PRC), and North Korea, and those countries’ nuclear coercion of U.S. allies and partners. Regional nuclear deterrence systems include the following:

• Dual-Capable Aircraft (DCA), including those operated by seven NATO allies as part of NATO’s “nuclear burden-sharing.” The last three NPRs have supported transitioning from the F-16 aircraft to the F- 35 Joint Strike Fighter. The Air Force is procuring the F-35 jointly with some NATO allies and other U.S. allies and partners. The DCA can carry the B-61 gravity bomb, which is deployed from some NATO bases under U.S. operational control.

• The U.S. Navy deploys a low-yield variant of the W76 warhead (the W76-2) on the Trident II D5 submarine- launched ballistic missile, as per a requirement articulated in the 2018 NPR.

• In 2024, the U.S. Department of Defense (DOD) initiated a program to procure a nuclear-armed sea- launched cruise missile as per Section 1640 of the FY2024 National Defense Authorization Act (P.L. 118-31). (See CRS In Focus IF12084, Nuclear-Armed Sea-Launched Cruise Missile (SLCM-N).)

U.S. Extended Deterrence and Regional Nuclear Capabilities

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Key Allied Relationships

Biden Administration officials have argued that U.S. allies and partners are a U.S. “asymmetric strategic advantage.” In a 2023 congressional hearing, former DOD officials stressed the importance of tending to the “software” of ally and partner assurance. Such efforts include declaratory policy by U.S. leaders, joint planning and close consultations, and exercises that enable allies to ascertain the credibility of U.S. defense commitments, including the potential U.S. use of nuclear weapons.

The Euro-Atlantic Region NATO’s nuclear policy has been changing since Russia’s 2022 invasion of Ukraine. According to NATO, “nuclear weapons are a core component of [the Alliance’s] overall capabilities for deterrence and defense, alongside conventional and missile defense forces.” U.S. strategic nuclear forces and independent U.K. and French strategic nuclear forces are described by NATO as the “supreme guarantee” of the security of NATO’s 32 member states. The United States also forward-deploys nuclear weapons for DCA delivery. (For more on NATO, see CRS Report R48121, NATO’s July 2024 Washington, DC Summit: In Brief.) The United States also has a separate mutual defense agreement with the United Kingdom that provides for military nuclear cooperation.

The Indo-Pacific Region U.S. allies Japan, South Korea (officially the Republic of Korea, or ROK), and Australia have expressed concern about changes in PRC and North Korean nuclear weapons and other capabilities. The United States maintains conventional military forces presence in Japan and South Korea, as well as elsewhere in the region, but it no longer forward-deploys nuclear weapons to these two countries’ territories. The 2018 NPR and Biden Administration officials have posited that SLCM-N would seek to deter adversaries and assure allies in the region.

In 1960, the United States signed a mutual defense treaty with Japan. The United States and Japan regularly engage and conduct alliance and defense cooperation, including through the Extended Deterrence Dialogue. (For more on U.S.-Japan relations, see CRS In Focus IF10199, U.S.- Japan Relations.)

In 1953, the United States signed a mutual defense treaty with the ROK. During the Biden Administration, the United States and the ROK created a Nuclear Consultative Group following the 2023 Washington Declaration. (For more on U.S.-ROK relations, see CRS In Focus IF10165, South Korea: Background and U.S. Relations.)

In 1951, the United States signed a mutual defense pact (ANZUS) with Australia and New Zealand. In the 1980s, the United States suspended its defense commitments to New Zealand following changes to New Zealand’s policies on nuclear weapons. (See CRS In Focus IF10389, New Zealand–U.S. Relations.) The U.S. and Australia’s mutual defense relationship continues; the two countries conduct a

Strategic Policy Dialogue. (For more, see CRS In Focus IF10491, Australia: Background and U.S. Relations.)

Other The United States has bilateral defense commitments with other countries that may or may not be included in the NPR’s “allies and partners” formulation for extended nuclear deterrence. The United States has an unofficial but robust relationship with Taiwan; the 1979 Taiwan Relations Act (P.L. 96-8) creates “strategic ambiguity” about potential U.S. actions in the event of a PRC attack on Taiwan. (See CRS In Focus IF12481, Taiwan: Defense and Military Issues.)

Issues for Congress

Congress provides authorization and appropriation of funds, as well as oversight of DOD, Department of State, and other U.S. government programs involved in carrying out U.S. extended deterrence policies. The Senate considers providing advice and consent to ratification of treaties negotiated by the executive branch. Members of Congress also directly engage with leaders in U.S. ally and partner countries. Through intelligence community briefings and other oversight activities, Congress may track changes in military capabilities of regional adversaries, including Russia, PRC, North Korea, and Iran, and assess indications that other countries, including U.S. allies and partners, are considering developing their own nuclear weapons.

Congress may oversee, including through hearings with executive branch officials and outside experts, U.S. efforts to assure allies and partners of the credibility of U.S. defense commitments. According to the 2023 final report of the Congressional Commission on the Strategic Posture of the United States, “any major change” to U.S. strategic policy or posture should be “predicated on meaningful consultations” with allies.

Congress may oversee the Administration’s assessment of the ways in which U.S. nuclear weapons fit into the overall mix of U.S. capabilities in regional security architectures. Some U.S. allies and partners in the Euro-Atlantic and the Indo-Pacific may request changes to existing U.S. extended nuclear deterrence commitments; U.S. allies and partners in the Middle East and elsewhere may request the protection of a U.S. “nuclear umbrella.”

Some Members of Congress cited concern about the potential insufficiency of U.S. regional nuclear capabilities when Congress required DOD to start up the SLCM-N program. Congress may continue to provide oversight of DOD and other U.S. government programs to modernize relevant U.S. nuclear and nonnuclear capabilities. Members of Congress may debate investments in, and deployment of, such capabilities.

Anya L. Fink, Analyst in U.S. Defense Policy

IF12735

U.S. Extended Deterrence and Regional Nuclear Capabilities

https://crsreports.congress.gov | IF12735 · VERSION 1 · NEW

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