When the leaders of China, Japan, and South Korea last met, in May 2024, observers viewed the meeting with a sense of relief. Japan and South Korea were emerging from one of the darkest periods in their bilateral relationship, when tensions over Japan’s colonial legacy in Korea had become so intense that they derailed traditional areas of cooperation in security and trade. In 2018, leaders in Tokyo reported that a South Korean warship had locked its radar on a Japanese patrol plane, and in 2019 the two countries launched a tit-for-tat escalation, in which Tokyo tightened export controls and Seoul responded by threatening to stop sharing intelligence. The substantive but fragile rapprochement between the two U.S. allies starting in 2022 allowed them to engage a powerful and assertive Beijing in a two-against-one dynamic.

Strengthening the Japanese–South Korean partnership is increasingly important as China flexes its muscle in the region. In November, Beijing launched a pressure campaign against Japan in response to remarks by Sanae Takaichi, the newly inaugurated prime minister, suggesting that Tokyo could get involved militarily if China were to attack or blockade Taiwan. China suspended seafood imports from Japan, canceled Japanese concerts and movie releases, and advised citizens against traveling to Japan. The Japanese Defense Ministry also reported that Chinese fighter aircraft had locked their radar on Japanese planes. It’s a familiar playbook for Beijing, which took many similar actions against South Korea when Seoul agreed to host a U.S. missile defense system, known as THAAD, in 2016–17. South Korea has so far stayed neutral in response to China’s pressure campaign against Japan, which reveals the lengths that Seoul and Tokyo still have to go before they can team up to counter Beijing’s coercion and deal with other pressing regional challenges, including newfound uncertainty about U.S. commitments and a strengthened axis of China, North Korea, and Russia.

The fate of Japanese–South Korean relations at this crucial moment may ultimately rest on the two countries’ new leaders, who, at first glance, do not seem to be natural partners. South Korean President Lee Jae-myung, who took over in June, has strong credentials on the South Korean left, which traditionally seeks engagement with Pyongyang—and, by extension, better relations with Beijing—and is less focused on strengthening the trilateral security partnership with Japan and the United States. South Korean progressives are also more inclined to call attention to Japan’s mistreatment of Koreans during its colonial rule and are willing to challenge existing bilateral arrangements for reparations. Meanwhile, Takaichi comes from the right wing of Japan’s dominant Liberal Democratic Party and represents the group most resistant to accommodating what it sees as South Korea’s continuous demands to address past injustices. She has visited the controversial Yasukuni Shrine, the memorial to Japanese soldiers that includes 14 convicted war criminals from World War II, and is a protégé of the former Prime Minister Shinzo Abe, whose revisionist views on history inspired deep antipathy among the South Korean public.

But this unlikely duo could be precisely the partnership needed to put the Japanese–South Korean relationship on a more resilient footing. The differing coalitions they represent will allow them to build a partnership with broader and more durable political support at home. Cooperation that takes root under this........

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