The U.S. and South Korean air forces training over South Korea’s West Sea, February 2023
The U.S. and South Korean air forces training over South Korea’s West Sea, February 2023
South Korean Defense Ministry / Handout / Reuters

Last month, South Korean President Yoon Suk-yeol warned that in the face of mounting North Korean provocations, South Korea might consider building its own nuclear weapons or asking the United States to deploy tactical nuclear weapons to the South, as it did during the Cold War. Although Yoon later stressed that his comments did not represent official policy, they were still significant—marking the first time that a South Korean president has raised the prospect of acquiring nuclear weapons since Washington withdrew its own nukes from the South in 1991.

Yoon’s comments are not the only indication that U.S. allies in East Asia are growing concerned about the credibility of Washington’s so-called extended deterrence commitments. These allies have little doubt that adversaries of the United States would think twice about attacking the U.S. homeland for fear of overwhelming retaliation; Washington is ably deterring that sort of aggression. They are increasingly uncertain, however, that U.S. deterrence extends to them. Would the United States honor its pledges to use the full range of U.S. capabilities, including nuclear weapons, to deter and, if need be, defeat external attacks on the territory and sovereignty of its allies?

In Japan, which has barred the possession, production, and introduction of nuclear weapons since 1971, discussions about acquiring a nuclear deterrent have not yet entered the mainstream. Nonetheless, unease is growing in the Japanese government about the ability of the United States to uphold its security commitments. In March 2022, former Prime Minister Shinzo Abe suggested that Japan should consider a nuclear-sharing arrangement like the one NATO maintains with Washington. Prime Minister Fumio Kishida quickly dismissed Abe’s suggestion, but last month, a meeting between Japan’s foreign and defense ministers and their American counterparts for the first time included a dedicated discussion of extended deterrence at this level, reflecting Japan’s heightened focus on the issue.

Worries about U.S. security guarantees extend beyond elite policy circles in Seoul and Tokyo. The Japanese and South Korean publics are also increasingly concerned about the credibility of U.S. extended deterrence and support moves to bolster their own countries’ defenses. A February 2022 poll by the Chicago Council on Global Affairs and the Carnegie Endowment for International Peace showed that over 70 percent of South Koreans support going nuclear and over 50 percent favor the return of U.S. tactical nuclear weapons to the peninsula. And according to a December 2022 poll conducted by the newspaper Yomiuri Shimbun, more than 80 percent of Japanese consider China, North Korea, and Russia as military threats. Strong majorities also support the Kishida government’s plans for unprecedented spending increases, underscoring the public’s concerns.

To assuage Tokyo and Seoul, the United States should make a concerted effort to shore up the credibility of extended deterrence. That means acknowledging the sources of allied fears, emphasizing that the presence of American troops in Japan and South Korea ties together the fates of the United States and its allies, increasing nuclear-related cooperation and planning with both countries, and possibly launching preliminary discussions about the potential redeployment of U.S. tactical nukes to South Korea. 


Allied apprehension about U.S. security commitments may seem puzzling in light of the Biden administration’s emphasis on shoring up U.S. alliances and the broad alignment of the Biden, Yoon, and Kishida administrations on North Korea and regional policies. But even at the best of times, maintaining the credibility of external deterrence commitments is a challenge. Doing so requires convincing allies not just that the United States has the capabilities to deter and defend against potential attacks against them but that it also has the will to use those capabilities—even if that means putting U.S. cities at risk. This is an inherently difficult endeavor, so a baseline of allied doubt about extended deterrence is predictable and normal.   

In recent years, however, the level of doubt among Japanese and South Korean officials has exceeded that normal baseline level. Rapidly intensifying threats—particularly from North Korea, China, and Russia—have created unique and urgent security challenges that Tokyo and Seoul worry the United States is either unwilling or unable to address. In 2022, for instance, North Korea embarked on an unprecedented campaign of weapons development, testing over 90 cruise and ballistic missiles of various ranges (one of which flew over Japan) and preparing for a nuclear test, which would be the seventh it has carried out since 2006. This turbocharged testing spree has enabled Pyongyang to advance toward its aim of credibly threatening to strike the U.S. homeland with a nuclear-tipped intercontinental ballistic missile, a capability that could sow doubt among U.S. allies about Washington’s willingness to put its own territory at risk to defend them. North Korea has also introduced a “first use” nuclear doctrine—threatening to use nuclear weapons in response to military interventions, regime-change efforts, or even new sanctions by outside powers—and is getting close to being able to deploy tactical nuclear weapons for use on the battlefield.

China’s sweeping military modernization is also fueling Japanese and South Korean security concerns. After more than two decades of near double-digit growth in defense spending, including efforts to improve its missile capabilities and dramatically expand its nuclear arsenal, China is increasing its maritime and air activity near Japan, including making regular incursions into the territorial waters around the Senkaku Islands (known in China as the Diaoyu Islands). China’s behavior has also increasingly unnerved South Korea as Beijing has come to see Seoul’s missile defense systems—including the Terminal High Altitude Area Defense (THAAD) battery deployed in 2017—as threats to China rather than as mere defenses against North Korea.

The war in Ukraine has provided North Korea with an ideal environment to accelerate its nuclear and missile programs.

Russia’s invasion of Ukraine and Moscow’s “no limits” partnership with Beijing have contributed to the uneasiness of Japan and South Korea, as well. For Tokyo, these developments have underscored the need to reinforce deterrence in the Taiwan Strait. Both Kishida and Yoon have drawn direct connections between the security of Europe and the Indo-Pacific, suggesting that Russia’s moves in Ukraine could spur China to attempt to take Taiwan by force or potentially tempt North Korea to invade the South. Japan’s new national security strategy, released in December 2022, notes that “Russia’s aggression against Ukraine has easily breached the very foundation of the rules that shape the international order” and that “the possibility cannot be precluded that a similar serious situation may arise in the future in the Indo-Pacific region.” Joint Chinese-Russian exercises near Japan last May and November further jarred Tokyo, as did China’s decision to fire five missiles into Japan’s exclusive economic zone during major military exercises around Taiwan in August.

For Seoul, the war in Ukraine has amplified the sense of vulnerability that comes with being a nonnuclear state next to nuclear-armed North Korea. Both Japan and South Korea worry that Russia’s aggression in Europe will distract the United States from the rapidly evolving array of security challenges in Asia. And Yoon recently pondered whether a prolonged war “could send a message to North Korea that the international community would fail to respond to an act of invasion with the appropriate sanctions or punishment, and that message would further encourage the North to conduct provocations.” The war in Ukraine has also provided North Korea with an ideal environment to accelerate its nuclear and missile programs, since Beijing and Moscow are no longer willing to join Washington and its allies in punishing Pyongyang for its provocations.  

Finally, U.S. allies in Asia are concerned about developments in the United States itself. In particular, they worry that a future U.S. administration could downgrade the importance of alliances, as U.S. President Donald Trump did during his administration. Both Tokyo and Seoul look at the lingering strain of “America first” thinking in U.S. politics and wonder whether Washington will remain committed to protecting them.


Japan and South Korea have responded to this heightened sense of vulnerability in part by increasing their own military capabilities. To enhance deterrence against North Korea, South Korea has developed a “three axis” defense system that consists of a preemptive strike platform, air- and missile-defense systems, and a retaliation plan designed to incapacitate the North Korean leadership and military facilities in the event of an attack. In order to fund this system and other defense capability improvements, South Korea has said it aims to increase defense expenditures by an average of 6.8 percent per year over the next five years, spending a total of $261 billion.

Japan has also taken bold steps to enhance its defense capabilities. It has committed to increasing defense spending by more than 60 percent over the next five years and to acquiring “counterstrike capabilities” that would enable it to strike North Korea or mainland China in the event of an attack. These pathbreaking steps, outlined in three strategic documents released late last year, represent the most dramatic advances in Japan’s defense capabilities since the end of World War II.

The United States has enabled these moves by its allies to take on greater defense responsibility while also reassuring them of the credibility of its security commitments. The Biden administration has supported the resumption and expansion of joint military exercises with South Korea, and in May 2021, Washington and Seoul announced the removal of restrictions on the range of South Korean missiles. The United States has also expressed support for Japan’s acquisition of “counterstrike capabilities” and is poised to sell U.S.-made Tomahawk Land Attack Missiles, which Japan plans to deploy on naval destroyers by 2026. And President Joe Biden has reiterated the United States’ commitment to defend its allies with the full spectrum of military capabilities, including nuclear weapons. He has also elevated extended deterrence dialogues with Japan and South Korea and encouraged greater missile defense cooperation among the three allies.

But these efforts, though important, have not been sufficient to extinguish allied concerns about extended deterrence. That is partly because reassurance alone can never be enough; nuclear commitments inevitably invite doubt that the United States would really risk a nuclear attack on one of its cities to defend Tokyo or Seoul. Moreover, the success that U.S. alliances have had in deterring a major war over the last eight decades ironically means that Washington’s security guarantees have never been put to a hard test.  

U.S allies also have bureaucratic reasons for their persistent doubts. Some U.S. officials have been reluctant to involve allies in the United States’ nuclear enterprise and operations much beyond occasional site visits. Some of this hesitancy stems from concerns about the security of classified information, but some of it stems from a general “just trust us” attitude with regard to nuclear weapons that has started to undercut the effectiveness of U.S. alliances. Extended deterrence dialogues with Japan and South Korea have become more detailed and comprehensive over time, and visits to facilities that support the U.S. nuclear umbrella offer some insight and assurance to U.S. allies. But they fall short of real transparency; there is a sense that the allies are little more than nuclear tourists.


Given these lingering doubts (and the costs that U.S. security guarantees entail for the United States), some analysts argue that Washington shouldn’t bother trying to salvage extended deterrence and that U.S. allies in East Asia should seek their own nuclear weapons. After all, this would lower some risks to the United States and allow highly capable allies to shoulder more of the responsibility for their own defense. Yoon has said publicly that South Korea could develop its own nukes quickly and with the support of its public. Alliance skeptics in the United States may even interpret such comments as further proof that the alliance system is outdated, costly, and dangerous because it has the potential to draw Washington into faraway conflicts.

Some U.S. commentators have wondered whether Washington ought to encourage U.S. allies to go nuclear. Such viewpoints are worthy of consideration, especially because they have gained adherents among advocates of restraint and isolationism on both the left and the right. But they rarely acknowledge all the problems that an indigenous nuclear program in South Korea would introduce—including forcing Seoul to withdraw from the Nuclear Nonproliferation Treaty and likely face sanctions, and risking a regional nuclear arms race between China, North Korea, South Korea, and possibly others. Indeed, a nuclear South Korea would almost certainly strain the U.S. alliance network in Asia, unleashing a divisive debate in Japan about that country’s military posture and the future of the U.S.-Japanese alliance, and creating additional friction in Tokyo’s relationship with Seoul. Australia would also likely worry about the destabilizing effect of a nuclear South Korea and the implications for other actors in the region, including Taiwan.

Those who advocate for downgrading or abandoning alliances would also need to detail an alternative method for managing the United States’ most serious geopolitical challenge—China—as well as issues such as climate change, pandemics, and supply chain problems that by their very nature cannot be handled by one country alone. Given that no such alternative has been offered, the United States will need to continue the difficult but necessary slog of shoring up extended deterrence, ensuring that this decades-old tool for maintaining regional peace and stability is up to the significant challenges of the coming decade.


To that end, Biden should build on the efforts his administration has already undertaken to restore the credibility of U.S. security guarantees. He should start by acknowledging the causes of allied concern: not just the increasing military capabilities of regional rivals but also the U.S. political realities that have caused allies to question Washington’s will to protect them from growing threats. Only by being more frank and open with allies—particularly regarding the nuclear aspect of extended deterrence—can the United States hope to assuage their fears.

At the same time, the Biden administration should emphasize that more than its credibility is at stake when it promises to use the “full range of U.S. capabilities, including nuclear,” to defend Tokyo and Seoul: more than 80,000 U.S. forces are stationed in Japan and South Korea, and 120,000 U.S. citizens live in the two countries, creating what the political scientist Joseph Nye calls a “community of shared fate” that binds the United States to its allies. This shared fate forms the core of extended deterrence and is a costly signal of U.S. resolve to deter and defend its allies against external aggression.  

To develop stronger bonds of trust, the United States should also create a framework for joint nuclear planning with Japan and South Korea similar to a NATO planning group, which would offer Tokyo and Seoul more insight into U.S. decision-making and processes involving nuclear weapons. Such a framework would allow the United States to plan bilaterally and trilaterally while remaining in control. The Biden administration might also consider augmenting existing bilateral extended deterrence dialogues by creating a new trilateral strategic dialogue to increase information sharing and identify coordination opportunities. If either Tokyo or Seoul is hesitant to participate, the exchange could be conducted at the so-called track 1.5 level between officials acting in an unofficial capacity and nongovernmental experts.

The United States should strengthen its regional military and nuclear posture as well, including possibly by doing more to showcase its nuclear capabilities. This could involve continually deploying submarines that can be equipped with nuclear missiles to the region or deploying aircraft capable of both conventional and nuclear missions on a rotating basis. The three allies should also explore opportunities for more joint military activities related to extended deterrence, such as combined Japanese and South Korean escorts of U.S. strategic bomber flights.

Finally, Washington should consider laying the groundwork for the possible redeployment of U.S. tactical nuclear weapons in South Korea. Under the present circumstances, the United States should not redeploy such weapons or support South Korea’s acquisition of them. But Washington and Seoul should consider beginning and publicly acknowledging planning for a potential future tactical nuclear redeployment to South Korea. This planning could include conversations about the infrastructure and military prerequisites for storing such weapons on the peninsula, as well as the potential downsides of doing so, including the high likelihood that tactical nukes would become targets for North Korean missiles, the potential for nuclear redeployment to heighten tensions in the region, the redundancy of tactical nuclear weapons given other U.S. capabilities, and the challenges these weapons could pose to controlling escalatory spirals in a crisis.

Before they make any decisions, the allies might also consider conducting war games to map out potential phases of preparation for redeployment, all contingent on changes in the threat environment. Such exercises would signal U.S. resolve and commitment to Japan and South Korea without crossing the proliferation threshold. They would also pressure Pyongyang to halt its provocations and perhaps spur China to rein in North Korea in order to prevent the return of U.S. tactical nuclear weapons to the peninsula. It is possible that Pyongyang would respond to planning of this kind with additional provocations, but the North could hardly be more belligerent than it was in 2022, when it conducted scores of missile launches. And in any case, the costs of North Korean provocations pale in comparison to the benefits of shoring up extended deterrence, which has underwritten 70 years of peace on the peninsula.

Allied concerns about U.S. credibility will never disappear entirely. But they can and should be managed to avoid the erosion of alliances. New levels of doubt among U.S. allies call for Washington to devise new ways to restore the credibility of extended deterrence—both to reduce Japan’s and South Korea’s sense of vulnerability and to ensure that U.S. alliances in Asia remain a force for stability long into the future.

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  • KATRIN FRASER KATZ is a Fellow at the Center for Strategic and International Studies and a Scholar in Residence at the University of Miami. From 2007 to 2008, she served as Director for Japan, Korea, and Oceanic Affairs at the National Security Council.
  • CHRISTOPHER JOHNSTONE is Senior Adviser and Japan Chair at the Center for Strategic and International Studies. At the National Security Council, he served as Director for East Asia from 2021 to 2022 and Director for Japan and Oceanian Affairs from 2014 to 2016.
  • VICTOR CHA is Senior Vice President and Korea Chair at the Center for Strategic and International Studies and a Professor at Georgetown University. From 2004 to 2007, he was Director for Asian Affairs at the National Security Council.
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