How Washington and Seoul Can Get on the Same Page to Deter North Korea

Moscow is using the threat of nuclear escalation to deter outside powers coming to Kyiv’s aid while it continues its devastating military assault on Ukraine. Not surprisingly, the Russo-Ukrainian War stimulated a boisterous debate in Seoul on how to make sure Kim Jong Un doesn’t follow Vladimir Putin’s playbook. Even though South Korea is a U.S. treaty ally covered by extended nuclear deterrence guarantees, which Ukraine is not, fears in Seoul of North Korean nuclear coercion are growing. The resulting policy debate centers on what steps the United States should take to increase the presence and visibility of nuclear weapons in the South Korean-U.S. alliance. As the Biden administration recalibrates policies to deal with the fallout from Russia’s invasion, addressing deterrence demands from allies in East Asia will be high on the list.

To deter North Korean nuclear coercion, and to disarm it in a crisis if needed, president-elect Yoon Suk-yeol has pledged to strengthen the South Korean-U.S. alliance and further develop capabilities for a pre-emptive strike. Some conservative politicians want Yoon to go further: to ask the United States to deploy nuclear weapons in South Korea or to “share” its nuclear weapons with South Korea, as it does with its NATO allies in Europe. These are not new ideas, but conservatives see the war in Ukraine as an opportunity to push Yoon to adopt them after he changed his position on nuclear weapons during the presidential campaign.

With a new Korean peninsula crisis looming, fueled by North Korea’s missile test and another possible nuclear test, the Yoon and Biden administrations ought to quickly get on the same page on how to deter North Korean nuclear coercion. The best way to prevent such coercion and to temper South Korean fears is for the Biden and Yoon administrations to make a high-profile announcement of new deterrence initiatives once the latter takes office on May 10.

Yoon inherits a robust defense budget and ongoing procurement of advanced conventional weapons. These include offensive strike missiles such as the Hyunmoo-4 and defensive systems such as the Korea Advanced Missile Defense and the U.S. Terminal High Altitude Aerial Defense (THAAD) medium-range missile defense system. In addition, South Korea has begun flying the F-35 fighter jet purchased from the United States. From a capabilities standpoint, South Korea appears well-prepared for a wide range of North Korea contingencies. Resumption of military exercises with the United States — another of Yoon’s pledges — will improve operational readiness.

However, enhancing the extended nuclear deterrence component of the South Korean-U.S. alliance presents challenges. Yoon seems poised to request a greater presence of U.S. nuclear weapons in the region and potentially even an unprecedented nuclear-sharing mechanism. The United States has a longstanding nuclear-sharing arrangement with several NATO states, which involves stationing U.S. nuclear weapons on their territories to be delivered by aircraft and pilots from those states in the event of a conflict. In some South Korean minds, these measures would make nuclear deterrence of North Korean threats more effective, thus dissuading Kim from attempting a Russia-style invasion.

The Biden administration should be receptive to ideas for enhanced consultation on extended deterrence, but is likely to deflect requests for new nuclear deployments in the region or nuclear sharing, which a senior U.S. State Department official stated in September 2021 “U.S. policy would not support.” To avoid an early misunderstanding with Yoon over a potential nuclear sharing or deployment request, the Biden administration should send prompt and firm signals that these measures should not top South Korea’s deterrence wish list and instead the two governments should pursue easier early wins.

For its part, the Biden administration clearly desires to engage South Korea in its “integrated deterrence” concept for the Indo-Pacific. This idea involves combining conventional, nuclear, cyber, space, and information warfare capabilities to deter a broader range of threats from adversaries. It is a pillar of the broader U.S. project of containing Chinese hegemony in Asia, and after reticence by the outgoing Moon administration to engage on these issues, Washington has high hopes that Yoon will be more amenable.

Although South Korean public opinion polls demonstrate increasing concerns about threats from China, Yoon presumably will tread carefully on signing up to the U.S. vision to avoid provoking Beijing early in his tenure. Recalling China’s damaging trade penalties on South Korea for agreeing to deploy the THAAD system in 2016, Yoon will need to walk a fine line between strengthening the alliance with Washington and refraining from jeopardizing deep economic ties with China.

Thus, there is a clear but narrow intersection of U.S. and South Korean interests on deterrence that suggests some low-hanging fruit for new initiatives. For example, the two sides could launch new high-level deterrence exercises (as a complement to regular military exercises) under the Security Consultative Meeting involving defense and foreign ministers. These exercises, to involve both senior military officers and government officials, could expand the range of contingency planning and simulate scenarios for North Korean attempts at nuclear coercion. Such exercises could also explore the conditions under which the United States would decide, with South Korean input, to use nuclear weapons against North Korea.

New deterrence exercises would be a significant step to strengthen the South Korean-U.S. alliance and signal resolve to North Korea and China. However, the peoples of South Korea and the United States are less likely to appreciate the importance of this step than are alliance managers within the two governments. To build greater public confidence about how the alliance enhances the security of both countries, senior South Korean and U.S. officials should engage in more public diplomacy on their approach to deterring North Korea’s nuclear coercion. Amidst high levels of public support for acquiring nuclear weapons in South Korea, such messaging would stress that the combination of South Korean military capabilities and U.S. extended nuclear deterrence is the best way to ensure South Korea’s security and demonstrate Seoul’s growing international clout. Although it would be unorthodox, the Russo-Ukrainian War demands more straight talk than usual: U.S. and South Korean officials should be ready to articulate why nuclear sharing or U.S. deployment of nuclear weapons are not necessary steps at this time.

With North Korea resuming a testing campaign for long-range missiles and possibly nuclear weapons, it is critical for the Yoon and Biden administrations to get out in front of a brewing crisis with actions to strengthen the alliance and messaging of solidarity. The best way to dissuade Kim from attempting nuclear coercion is to demonstrate why it will fail.

Toby Dalton is senior fellow and co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace.

Image: Republic of Korea air force photo by Chief Master Sgt. Kim, Kyeong Ryul