Paper Tigers and Iron Domes: The Uncertain Future of Extended Deterrence in the Indo-Pacific

The chaotic opening of the second Trump administration has sent shockwaves through U.S. alliances and reignited debates about the status of the U.S. military presence abroad. Trump’s transactional approach to alliances and engagement with adversaries could negatively impact extended deterrence in the Indo-Pacific region, with key allies, mainly Japan and the Republic of Korea (ROK). As a result, there has been renewed discussion about whether Japan and the ROK should reconsider their pledge not to pursue their own nuclear weapons programs. Even as the ROK and Japan contend with an increasingly unreliable United States that may not provide the security they need, pursuing nuclear weapons will only exacerbate the tense regional security environment and reduce overall security. Establishing their own nuclear capabilities would threaten their bilateral relationships with the United States, risk a volatile regional arms race, and hurt both countries’ reputations as nonproliferation advocates. 

Extended Deterrence in the Indo-Pacific

Extended deterrence and the United States’ nuclear umbrella are foundational elements of the United States’ security relationships with Japan and the ROK. Extended deterrence relies on the threat of a massive retaliatory response in the event of attacks on U.S. allies. The 2022 U.S. Nuclear Posture Review, which outlines the administration’s nuclear policy, describes extended deterrence as contributing 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.” Both countries are signatories to the Non-Proliferation Treaty, committing them to not pursue their own nuclear weapons programs. While the nuclear umbrella is the cornerstone of extended deterrence, the United States’ significant military presence in Japan and the ROK contributes to the deterrence mission.

Since the end of the Korean War, the United States has maintained a significant military presence in the ROK, including 24,000 deployed troops under U.S. Forces Korea. While minor reductions in troop levels have occurred over the decades since the Korean War, the persistent threat posed by North Korea has led to this continuous military presence. Over twice as many U.S. troops are stationed in Japan under U.S. Forces Japan, the largest overseas deployment in the Indo-Pacific. U.S. bases in the ROK and Japan are among the most important in the region, with Yokosuka Naval Base in Japan serving as the headquarters for the U.S. 7th Fleet. These bases are critical for U.S. power projection in the Indo-Pacific and for countering the People’s Republic of China’s (PRC) regional dominance. Mutual defense treaties between the U.S.-ROK and the U.S.-Japan further strengthen the U.S. defense relationship with these nations and have served as foundations for the development of their diplomatic and economic relationships with the U.S.  In large part because of these commitments from the United States, Japan and the ROK have not developed their own nuclear weapons capabilities.

Why Domestic Nuclear Weapons Programs Harm Japanese and ROK Security 

With the United States’ turn toward isolationism and President Trump’s unpredictable approach to U.S. alliances, nuclearization may seem like an attractive option for the ROK and Japan. However, the development of their own nuclear weapons programs could further destabilize the already tense regional environment. First, pursuing nuclear weapons could accelerate the deterioration of the U.S.-ROK and U.S.-Japan alliances. Both alliances have deep historical roots. Going back on their commitment to the United States not to pursue nuclear weapons would damage both countries’ credibility and bilateral relationships and give Trump fuel to divest from their defense. Second, it could launch a regional arms race. Third, it would erode both states’ international reputations as members of the Non-Proliferation Treaty (NPT) and could prompt other small non-nuclear weapons states to pursue nuclear capabilities.

The U.S.-ROK alliance originated in the aftermath of the Korean War. Initially designed to counter the threat posed by the DPRK, the alliance has since evolved into a robust strategic partnership. The United States sustains a presence of over 28,500 military personnel, conducts annual joint military exercises, and sells military equipment to the ROK to boost its defense modernization. These elements contribute to the ROK’s defense posture against both the DPRK and the PRC. The partnership also serves as a key component of the United States’ regional alliance network that is critical in countering the PRC’s aggressive actions and projecting American power in the Indo-Pacific region. In 2023, in response to concerns from the ROK about the U.S. commitment to Korea, the United States and the ROK signed the Washington Declaration establishing the U.S.-South Korean Nuclear Consultative Group. The United States agreed to deploy nuclear-armed submarines to the ROK periodically and include the South Koreans in the United States’ nuclear planning for the region. In exchange, the ROK committed to not developing its nuclear capabilities. Breaking this agreement so quickly after its establishment would likely threaten the stability of the U.S.-ROK security partnership. 

The U.S.-Japan alliance, initially imposed on Japan as a condition for the end of U.S. military occupation following WWII, has evolved into one of the core aspects of U.S. strategic posture in East Asia. While the JSDF is a powerful military force, its reliance on U.S. forces for decades has left it with key weaknesses, most notably its size and ability to conduct joint operations. The Japanese Constitution “renounces war” as a tool of the state and thus significantly limits the role of the JSDF to solely defensive activities. Due to Japan’s experience in WWII and the following rise in pacifism, the Japanese public historically held negative views of the JSDF. Public opinion has warmed in the decades since, but the JSDF’s mission as a purely defensive force has resulted in a lag in recruitment and development of advanced weapons capabilities. The JSDF has fewer servicemembers than its regional peers; roughly 250,000 active-duty personnel comprise the JSDF, whereas the ROK has over 500,000 active-duty personnel. The ground, air, and maritime services are not well-integrated to conduct joint operations across multiple domains. In large part due to growing regional threats from the PRC and the DPRK, Japan has focused more resources in recent years on building up its national defense capabilities. Pursuing a nuclear weapons program could drive the United States away from the alliance, leaving Japan even more vulnerable than it was before.

Japan and the ROK pursuing nuclear weapons could trigger an unpredictable arms race that would increase regional instability. Both the PRC and the DPRK have made significant moves to develop their nuclear capabilities. In 2021, open-source imagery intelligence in China revealed a massive nuclear modernization campaign through the construction of hundreds of new missile silos intended for ICBMs. The DPRK has grown increasingly aggressive in demonstrating its nuclear capabilities and has accelerated its ballistic missile tests in recent years; its most recent test of an ICBM was conducted just days before the 2024 U.S. election. The PRC and the DPRK have cited countering perceived threats from the United States as a key motivator in their nuclear modernization efforts. Both countries would likely view Japan and the ROK pursuing nuclear weapons as a threat. This would provide them with a justification for an even more rapid expansion of their capabilities or other aggressive behavior.

If the ROK and Japan pursued nuclear weapons, they would undermine the entire non-proliferation regime. It would weaken the ROK and Japan’s international reputations as members of the NPT and could prompt other non-nuclear weapons states to pursue nuclear programs. As the only country to have experienced a nuclear detonation on its territory, Japan has been a staunch advocate of global nuclear disarmament. The Three Non-Nuclear Principles serve as the foundation of Japan’s nuclear policy and commit it to not possessing or manufacturing nuclear weapons, nor allowing them on Japanese territory. The ROK made similar commitments in the Joint Declaration on the Denuclearization of the Korean Peninsula in 1992 and has since relied on its non-nuclear status when condemning the DPRK’s nuclear program. A reversal of either state’s nuclear policies would erode their international legitimacy and the global nonproliferation regime. It would also give the PRC and North Korea rhetorical leverage by claiming that the United States, Japan, and the ROK are escalating tensions and creating instability in the region, thus justifying their own nuclear build-ups. 

The economic costs of pursuing a nuclear weapons program would also likely be significant, as both international agreements and U.S. law could lead to the imposition of sanctions and other restrictions. While it is possible that the United States could offer some sanctions relief, this would be far from certain, especially if nuclearization were carried out without U.S. consultation or approval. Even if the United States blocked some sanctions, key trade partners such as the PRC and the EU would also be able to apply significant economic pressure. The sanctions levied against Iran and the DPRK for their nuclear programs decimated their economies. The ROK and Japan are far more integrated into the global economy and thus would likely suffer more. 

In addition, Japanese or South Korean nuclearization could jeopardize Trump’s arms control plans with Russia and the PRC, which would strain their bilateral relations with the United States. Since assuming office in January, Trump has called for denuclearization and arms control efforts with Russia and the PRC as a way to reduce military spending. While it is unclear how viable these efforts are, Russia or the PRC could use Japanese or South Korean nuclear ambitions as an excuse to back out of arms control talks with the United States, arguing that they should not have to play by the rules if U.S. allies are not. Alternatively, Russia and China could use Japan or the ROK’s nuclear ambitions as a bargaining chip in arms control talks in an attempt to gain concessions from the United States that are favorable to their positions. Either way, the pursuit of nuclear weapons by key U.S. allies would complicate any already complex trilateral arms control efforts between the great powers. Unrestrained Russian and Chinese nuclear arsenals would not serve Japan’s or the ROK’s security interests.

Extended Deterrence Remains an Essential Ingredient for Security  

While Japan and the ROK’s security concerns are valid in an increasingly unpredictable geopolitical environment, developing their own nuclear weapons will ultimately make them less secure. The long-term consequences of pursuing nuclear weapons for both countries are not worth the potential short-term gain in security. Japan already possesses nuclear latency—the technical ability to quickly develop a nuclear weapon—derived from its advanced civilian nuclear power program, and South Korea is considered to be on the cusp of latency as well. Should either state cross the line into military nuclear capabilities, it would be incredibly difficult to walk that move back and would cause irreparable damage to their global reputations and international relations. 

Extended deterrence remains the best option for both Japan and the ROK to maintain their security. To maintain their respective relationships with the United States, both countries should focus on developing their conventional forces so they become too valuable as regional partners for the United States to abandon. Trump has recently taken aim at Japan for what he’s labeled a “one-sided” bilateral relationship, yet Japan contributes substantially to its security cooperation with the U.S., including providing extensive strategic basing for U.S. troops and $1.69 billion to maintain these bases. The ROK also supports the U.S. military presence on its territory, recently increasing its planned annual financial contributions to $1.13 billion for 2026.  This presence allows the United States to keep a close eye on the DPRK and serve as a deterrent against aggression. Japan and the ROK must highlight these contributions so the Trump administration understands the consequences of abandoning these alliances. 

Views expressed are the author’s own and do not represent the views of GSSR, Georgetown University, or any other entity. Image Credit: United States Forces Korea