Skand Tayal | Will Unilateralism by Trump’s US Drive Japan & South Korea to ‘Nuclearisation’?

While successive US administrations have assured Japan and South Korea that its nuclear umbrella over its allies is “ironclad”, President Donald Trump’s recent moves in practically abandoning Ukraine to its own fate may force the strategists of the two Northeast Asian democracies to wonder whether a future US President would be ready to risk an attack on California to save Tokyo or Seoul.

This uncertainty is of existential significance to policymakers of both Japan and South Korea. As global strategic power shifts towards the Indo-Pacific with China seeking to dominate the region,

Japan is pressed between two conflicting imperatives: growing regional insecurity and a post-war identity built on pacifism and nuclear restraint. The intensifying rivalry between the US and China, North Korea’s mercurial behaviour and the fate of non-nuclear nations like Iraq, Libya, Syria and Ukraine is likely to trigger a debate about whether Japan should chart its own nuclear course.

The regional security environment has darkened since President Xi Jinping consolidated his power in the last decade. China’s assertiveness has moved beyond the realm of economic coercion such as restrictions on rare earth minerals to Japan to hard military posturing. Beijing’s expanded naval presence near Japan’s Senkaku Islands, live-fire exercises around Taiwan and a burgeoning blue-water navy signal a state preparing for territorial expansion by force. Simultaneously, North Korea’s continued missile testing and unrelenting nuclear development intensify the threats facing Japan and South Korea. Compounding this is the breakdown in the international rules-based order reflected in Beijing’s defiance of UNCLOS in the South China Sea and Russia’s “special military operation” against Ukraine.

Amidst this volatility, the credibility of the extended deterrence by the US, a cornerstone of Japanese security, is bound to come under scrutiny. The first Trump administration’s transactional approach to alliances and public musings about troop withdrawals had seeded doubt in Tokyo. While the Biden administration had moved to reinforce alliance structures, the spectre of a return to American isolationism has again raised questions about how much Japan can depend on others for its defence.

Despite this uncertainty, nuclearisation remains politically unpopular in Japan. Former Prime Minister Fumio Kishida, a native of Hiroshima, had reaffirmed Japan’s adherence to its three non-nuclear principles: not to possess, produce, or permit introduction of nuclear weapons. Even conservative analysts who advocate revision of Japan’s Constitution, expanding the definition of “self-defence” and increased military budgets, stop well short of calling for indigenous nuclear arms.

The ruling side is testing the boundaries of Japan’s pacifist Constitution-- doubling the defence budget and acquiring long-range strike capabilities -- but it has not ventured into nuclear territory.

Tokyo’s response to the deteriorating security environment in the Indo-Pacific in the face of an expansionist China has been robust, but not nuclear. It's National Security Strategy and 2024 Defence White Paper mark a decisive shift toward hardening its defence posture; acquiring counter-strike capability, strengthening missile defence and operationalising new domains like cyber and space. It is on course to become a strong conventional military power without abandoning its anti-nuclear commitments.

But this sentiment can change, just as it has changed in India. Japan possesses the technical capacity to build nuclear weapons. Its advanced civilian nuclear infrastructure, scientific base and delivery capabilities place it among the most prepared non-nuclear states in the world. Under the civil nuclear cooperation agreement with the US in 1987, Japan can reprocess spent fuel from its civil nuclear reactors. It, therefore, has a stockpile of plutonium, which can be diverted to form plutonium bombs.

On the other side of the East Sea, South Korean policymakers have been debating for decades its possible nuclearisation as a response to the irreversible and growing nuclear weapon programme and delivery system of North Korea.

In January 2023, former President Yoon Suk-yeol said that if Pyongyang’s provocations intensify, “it is possible that our country will introduce tactical nuclear weapons or build them on our own”.

Responding to this pro-nuclear sentiment, in April 2023 the US signed the “US-ROK Washington Declaration”, strengthening the nuclear deterrent cooperation by more consultation on strategic nuclear matters. But that was under the Biden administration.

While 2018-19 witnessed an unprecedented thaw in relations between the two Koreas, the North Korean rhetoric has now hardened. In October 2024, North Korea amended its constitution declaring South Korea a “hostile state”, and gave up its long-lasting policy of “reunification” with South Korea.

Part of this complex riddle is the continuing US refusal to allow South Korea to reprocess its spent nuclear fuel from nuclear power stations. Under the 2015 “US-ROK Agreement for Peaceful Nuclear Cooperation”, South Korea’s reprocessing is subject to further negotiations with the US. South Korea has a huge and mounting stockpile of highly hazardous spent fuel. Its ambassador to the US had in October 2024 reportedly said this issue would again be discussed with the incoming US administration.

Much will depend on the Trump administration’s future strategy. President Trump advocates “burden sharing” with its allies and has encouraged regional powers to take more responsibility for their own as well as regional security. But what are the limits of this approach? And how dependable are the assurances of the United States where the occupant of the White House changes every four or eight years? Tokyo and Seoul need to ponder over these uncertainties to address this existential question.