Japan and South Korea Grow Closer

The Trump effect on Japan–South Korea relations

By Daniel Sneider, lecturer in East Asian studies at Stanford University, non-resident distinguished fellow at the Korea Economic Institute of America, and APP Member. 

First Published July 12, 2026 at the East Asian Forum.

The August 2023 Japan–South Korea–US trilateral summit at Camp David provided powerful impetus for Tokyo and Seoul to restore warm ties, an undertaking initiated by former South Korean president Yoon Suk-yeol.

The momentum in Japan–South Korea relations has continued, surviving changes in leadership in all three countries. Even the potentially disastrous pairing of a conservative nationalist government in Tokyo and a progressive administration in Seoul has not slowed the pace of the bilateral partnership’s deepening.

Japanese Prime Minister Sanae Takaichi and South Korean President Lee Jae-myung sit at two ends of the political spectrum, but their May 2026 meeting in Lee’s hometown of Andong on 18-19 May, was marked by a spirit of warmth. Even Lee’s critics, who feared that his long-held harsh views on the legacy of Japanese colonial rule would reverse progress, were pleased.

Former Korean Ambassador to Japan Shin Kak-soo was ‘quite impressed with how [Lee] handled South Korea–Japan relations after taking office’, noting that the meeting in Andong ‘went very well’ — a view shared by senior South Korean officials in off-the-record discussions.

This success can be credited to Japan and Korea’s shared challenges of hedging against both China and an unpredictable United States, with US President Donald Trump’s trade wars and military interventions sending shocks across both economies. Lee’s decision to avoid past sources of contention like wartime history issues with Japan can be read as a calculated effort to use closer ties with Tokyo to insulate South Korea from growing chaos in an unpredictable security environment.

The most powerful factor in this is the prospect of US withdrawal from global leadership under Trump 2.0. Another South Korean senior official noted off the record that South Korea and Japan have ‘shared anxieties about Washington’ and that ‘it is not about China’.

Senior South Korean officials are discussing the need for closer security ties with Japan in response to US retreat, including potentially finessing the territorial disputes by signing a non-aggression pact and moving towards a mutual defence treaty. While this would go beyond what most South Koreans would tolerate — and much less in Japan — its mere consideration is a remarkable sign of the impact of Trump-driven uncertainty.

This tightening is taking place without active US involvement at the highest level. While trilateral meetings are still being held among senior officials and statements are being issued with common views on regional and global security issues like North Korea, this is the product of institutional inertia created by former US president Joe Biden’s administration. There is no evidence of a connection between those official pronouncements and the White House and Trump has never expressed any interest in trilateral cooperation, nor is he eager to follow in Biden’s footsteps.

Despite the factors impelling closer ties, there are serious concerns in Seoul and Tokyo that the rhetoric of bilateral partnership has not been sufficiently reflected in concrete actions.

The most cited example of this is the potential admittance of South Korea to the Comprehensive and Progressive Agreement for Trans-Pacific Partnership, a move which could forge closer trade and investment alliances and counter US mercantilism. While Seoul has expressed its interest in joining, it has yet to formally apply. Domestic politics remain a constraint, reflected in the ongoing restrictions on the purchase of Japanese seafood due to the Fukushima nuclear accident and its aftermath.

A similar dynamic is visible in the arena of defence cooperation. Japanese Defence Minister Koizumi Shinjiro visited Seoul in June 2026 and reaffirmed tightening defence ties and trilateral cooperation with the United States. But he was unable to get Seoul to sign off on a long-sought military logistics pact which would institutionalise joint operations. Lee told reporters earlier in June that public sentiment on history issues were the obstacle to signing what would otherwise be a logical agreement.

Beyond immediate issues, two policy areas — China and North Korea — could also drive the neighbours apart.

Lee has been careful to maintain good ties with Beijing, both in the economic realm and in potentially seeking China’s help in dealing with North Korea. The divergence between Lee’s policy towards China and the hostile state of Japan–China relations was noted at the Andong meeting.

Both governments share a commitment to the eventual denuclearisation of the Korean peninsula. But Lee seems ready to back a possible Trump summit with North Korean Supreme Leader Kim Jong-un which could offer de facto acceptance of North Korea’s nuclear status. Japan has always been fearful that this outcome could lead to a separation of the Korean peninsula from US extended deterrence commitments and leave Japan exposed to Pyongyang’s nuclear threat.

The relationship also remains marred by the lingering issues of wartime history and justice that have been carefully put aside but not resolved. Compensation cases against Japanese companies for wartime workers in Japanese mines and factories continue to make their way through South Korean courts. Takaichi has carefully avoided doing anything to inflame Seoul, but it would only take a visit to the controversial Yasukuni Shrine — something she did regularly for several decades — to relight the fires of South Korean patriotism.

For now, the path to improved ties looks open, thanks in large part to Trump and to a lesser extent to Chinese President Xi Jinping. But the question of how long it will remain so is far less certain.