Japan-ROK-US Trilateral Relations: Flying into Turbulence

The trilateral partnership between Japan, South Korea (ROK), and the US has progressed significantly since it was announced in August 2023. Today, the trilateral is wide-ranging – with cooperation in the economics, defence and technological spheres, fronted by messaging on promoting a rules-based regional order. However, with political ructions in South Korea and Japan, and the return of Donald Trump to the Oval Office, the trilateral is flying into some turbulence. This will have impact on Southeast Asia.

There are substantial stakes here. The Biden administration has been criticised by some in Southeast Asia for its approach in the economic realm, including its inability to gain traction with Southeast Asia and the wider Indo-Pacific. Nevertheless, the outgoing administration’s signature achievement has been in the realm of bolstering alliances and partnerships to promote the regional order.

In their landmark Spirit of Camp David statement, Japan, the ROK and the US had agreed to enhance cooperation to promote security and prosperity in the Indo-Pacific. Seoul and Tokyo reached this breakthrough despite their bilateral historical grievances which have bedevilled relations. This development and the separate US-Japan-Philippines trilateral announced in April 2024 are historic, showing that the US is seeking to share the burden of regional security with trusted allies. It is uncertain whether Trump will be content with the first trilateral arrangement. It is quite likely that Trump will, as he did in his first term as president, press South Korea and Japan to bear more of the collective defence burden. That said, if the two countries do so, the trilateral arrangement could survive a second Trump administration.

In another first, the three countries said in November 2024 that they would institutionalise their trilateral by establishing a secretariat to coordinate and implement their shared comments and a three-way security cooperation framework to institutionalise cooperation among their defence authorities. Their goal is clear: to uphold the status quo in the waters of the Indo-Pacific amid growing assertiveness from China.

Unfortunately, this three-legged stool is looking wobbly given recent events.

On 14 December, South Korea President Yoon Suk-yeol was impeached by lawmakers following his declaration of martial law on 3 December 2024. This raises questions about the longevity of his policy agenda and whether his successor would maintain the same commitment to the trilateral, its current Indo-Pacific strategy, and the Korea-ASEAN Solidarity Initiative (KASI). A Democratic Party win in South Korea’s next election could shift foreign policy toward the party’s long-standing stance which includes balancing ties with Washington and Beijing, engaging North Korea, and adopting a more critical stance on Japan and the US.

Similarly, Japanese Prime Minister Shigeru Ishiba’s loss of a majority in the Lower House following the October parliamentary elections introduces potential uncertainty about his government’s ability to fully implement its foreign policy agenda. This political shift could complicate Japan’s trilateral cooperation with the US and South Korea, particularly in addressing shared challenges posed by China, Russia, and North Korea. While the core principles of Japan’s foreign and security strategy are likely to remain stable, these domestic dynamics may affect the pace and consistency of policy execution, introducing uncertainties into the Japan-ROK-US alignment at a critical time.

The return of Trump to the White House adds another layer of complexity. During his first presidential term (2017-2021), Trump’s preference for bilateralism over multilateralism unsettled US allies. His transactional approach to defence partnerships created doubts about Washington’s commitment to collective security frameworks, and undermined trust and collaboration. Trump’s insistence on increased host-nation support, such as demanding a 400 to 500 per cent increase in financial contributions from South Korea and Japan for hosting US troops, caused friction in both capitals. Similar demands could resurface next year.

…The silver lining is that the strategic imperatives behind the Japan-ROK-US trilateral would likely outlive these two leaders.

While the implications for a troubled US-ROK-Japan trilateral for Southeast Asia may not be directly evident, developments within the trilateral could have significant ramifications: mainly, any weakening of the trilateral could embolden Beijing to assert its influence more aggressively and not just in Northeast Asia. This would undermine Southeast Asia’s maritime security, as this region has long relied on US-aligned partners’ presence and support. Japan and South Korea have acted as auxiliaries of Washington to help Southeast Asian states at the frontline of confronting China in the South China Sea.

Japan is a trusted economic and security partner for Southeast Asian countries. It has contributed coast guard ships to the Philippines and Vietnam, and in 2023, it introduced its Official Security Assistance (OSA) programme, which provides equipment and infrastructural assistance to recipient countries in Southeast Asia, to improve their deterrent capabilities.

South Korea has a less prominent role than Japan but has made notable strides in engaging the region. It has participated in commander-level discussions and provided coast guard training to some ASEAN countries, ranking fifth in its participation in regional military exercises, after second-placed Japan. While Japan and South Korea play independent roles, they enable the Japan-ROK-US trilateral to be involved in enhanced security cooperation with Southeast Asia.  

At this juncture, President Yoon and Prime Minister Ishiba are politically enfeebled and their successors would likely struggle to continue the two countries’ security efforts in Southeast Asia. However, the silver lining is that the strategic imperatives behind the Japan-ROK-US trilateral would likely outlive these two leaders and will even endure the vicissitudes posed by another Trump term. In an era of straitened resources, burden-sharing among allies and partners makes sound strategic sense.

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