How to Manage a Patron’s Unpredictability? Japan and South Korea React to the New United States Administration

CSDS POLICY BRIEF • 32/2025

By Mina Pollmann

17.12.2025

This CSDS Policy Brief has been produced in collaboration with The Hague Centre for Strategic Studies, as part of the PROGRESS 3.0 project.

Key issues

• Japan and South Korea both have higher abandonment concerns, are facing greater burden-sharing pressure and are diplomatically diversifying;
• Japan’s entrapment concerns are lower than South Korea’s because Japan sees defending Taiwan as more in its own interest than South Korea does;
• The United States’ (US) unpredictability accelerates trends in Japan towards investing more in its own self-defence, and South Korea’s reaction to the US administration is constrained by the North Korean threat.

Introduction

Unpredictability in US policy towards adversaries such as China, Russia and North Korea in the early months of the second Donald J. Trump administration caused anxiety among US allies about the future trajectory of US foreign policy.

The US National Security Strategy (NSS), released in early December 2025, prioritises homeland defence and intervention in the Western hemisphere. On the question of Taiwan, the NSS states, ‘deterring a conflict over Taiwan, ideally by preserving military overmatch, is a priority. We will also maintain our longstanding declaratory policy on Taiwan, meaning that the United States does not support any unilateral change to the status quo in the Taiwan Strait’. Furthermore, it calls on the United States, alongside US allies and partners, to jointly acquire the capabilities to ‘deny any attempt to seize Taiwan or achieve a balance of forces so unfavorable to us as to make defending that island impossible’. The debate over the NSS’s position on China was likely a cause of the delay for publication. Though the NSS stakes out the US position, it is not settled. Divisions within the administration are likely to persist and given President Trump’s focus on a trade deal with China and economic advantage more generally, he may walk back his commitment to Taiwan later.

US policy towards Russia is even more opaque. The NSS declares ‘it is a core interest of the United States to negotiate an expeditious cessation of hostilities in Ukraine, in order to stabilize European economies, prevent unintended escalation or expansion of the war and reestablish strategic stability with Russia’, but it stopped short of criticising Moscow. President Trump held the first US summit meeting with Russian leader President Vladimir Putin since the Russian invasion of Ukraine in 2022 in Alaska in August 2025. There is an “on-again, off-again” dynamic to his proposed second summit with President Putin in Budapest, with unanswered questions about how much pressure President Trump is willing to put on President Putin to end the war in Ukraine – or on Ukrainian leader President Volodymyr Zelensky. The recent round of peace talks in Florida is ongoing at the time of writing. After President Trump withdrew from the Intermediate-range Nuclear Forces Treaty (INF) in his first term, New START is the only nuclear arms pact between Washington and Moscow. Though New START has been inoperative since the invasion of Ukraine, Russia nominally pledged to respect its limits until this past August. It is set to expire in February 2026, but President Putin proposed extending it for another year. The US has not finalised its response yet, though it must carefully consider what it will gain and what implications it will have for how the Ukraine war is resolved.

US policy on North Korea was and continues to be a source of extreme uncertainty. The NSS does not even mention North Korea and the US bureaucracy has already had to walk back multiple comments by President Trump that North Korea is a nuclear weapons state in his second administration. President Trump’s three summits with North Korean leader Kim Jong Un in his first administration did not produce any concrete results. Yet President Trump is still interested in speaking to Kim again and allies and analysts watched with bated breath to see if there would be a sudden summit announcement during his trip to the region in late October 2025. Furthermore, there has been increasing Sino-Russian activity in the Pacific and Arctic – including in the Sea of Japan. As North Korea’s support for Russia’s war in Ukraine grows, North Korean soldiers are gaining battlefield experience and Russia is offering North Korea greater military technology support.

How is this unpredictability affecting how Japan and South Korea – two key US allies in the Indo-Pacific – perceive the US as an ally? How are they responding diplomatically? As this CSDS Policy Brief shows, so far, Japan and South Korea both have higher abandonment concerns under President Trump and are responding to increased US pressure to burden-share. Japan has low entrapment concerns, while the US focus on Taiwan might be increasing entrapment concerns in South Korea. Both Japan and South Korea are diplomatically diversifying by investing in their security relationship with other US allies and partners. Japan and South Korea are both moving slowly and cautiously in response to the new administration, trying to appease the US at the same time as laying the foundation for a more independent foreign policy and self-defence capability in the long term.

Japan’s threat perceptions

China presents a direct and multifaceted threat to Japan. Japan and China have a territorial dispute over the Senkaku/Diaoyu islands, in the East China Sea, where China has been increasing its military activities. Japan’s degree of involvement in the US defence of Taiwan is also a major point of friction, as seen in the recent diplomatic spiral following Prime Minister Takaichi’s remarks about a “survival-threatening situation” in a Diet meeting on 7 November 2025. Though Prime Minister Takaichi is strongly pro-Taiwan, Japan has also traditionally maintained strategic ambiguity over Taiwan. While many Japanese strategic thinkers would agree that if the US were to defend Taiwan in a contingency, this would constitute a “survival-threatening situation” for Japan – because of Japan’s reliance on the US for its security, dependence on secure maritime trade routes and proximity of Japanese islands to the Taiwan Strait –, previous prime ministers kept their statements vaguer. China is unlikely to attempt a direct takeover of the Senkaku/Diaoyu islands. The more likely scenario that Japan is preparing for is Chinese attacks on US Forces Japan (USFJ) or Japanese Self-Defence Forces (JSDF) to pre-empt or prevent US or Japanese involvement in a Chinese attack on Taiwan. Finally, as an island nation that relies on international trade and the security of maritime shipping routes for its prosperity, Japan is a champion of the rules-based international order that Chinese military activities in the Taiwan Strait, East China Sea, South China Sea and beyond undermine.

Russia also threatens Japan directly from the north and indirectly through its assault on the rules-based international rules. Under the slogan “Ukraine today may be East Asia tomorrow”, Japan has been a staunch supporter of Ukraine since 2022. Japan’s initial interest may have been driven by an abstract concern about upholding the rules-based international order, a desire to stimulate Japan’s defence consciousness, a need to make sure that China did not learn the “wrong lessons” from the Russian experience and a plan to draw Europe’s attention to East Asian security issues. Japan also has a territorial dispute with Russia over the Northern Territories/South Kuril Islands. Takaichi has indicated that she would like to resolve the issue and sign a peace treaty with Russia. In the long term, Japan is concerned about the Arctic as an arena of competition and possible Russian retaliation for Japan’s support of Ukraine (e.g. increased support for China).

Japan is very concerned about North Korea’s missiles and nuclear weapons development. A US deal with North Korea in which Pyongyang gives up its intercontinental ballistic missiles (ICBMs) but is allowed to keep its short- and medium-range missiles is Japan’s worst nightmare. US recognition of North Korea as a nuclear weapons state would not change facts on the ground but would undermine the credibility of the Non-Proliferation Treaty (NPT) regime. During President Trump’s first administration, Japan supported US engagement policy to maintain alignment with the United States, but generally prefers a policy of maximum pressure on North Korea. In addition, Japanese abductees continue to be a domestically salient issue. Prime Minister Takaichi has pledged to resolve this issue and suggested a face-to-face meeting with Kim Jong Un to do so. Prime Minister Takaichi arranged for President Trump to meet with family members of abductees during his visit to Tokyo in October 2025. Given the longstanding stalemate on the issue, there is some hope that President Trump’s erratic style of diplomacy may be what is needed to create momentum on this issue. Despite colourful, anti-Japanese rhetoric, the real North Korean danger to Japan is not a random attack on Japan, but Japan being targeted to pre-empt or prevent JSDF supporting USFJ and US Forces Korea (USFK) and/or USFJ defending South Korea alongside USFK and the South Korean military.

Japan’s response

Despite many legal and institutional constraints, Japan is continuing a trend that preceded President Trump by investing in its own defence capabilities. The cabinet approved ¥847.2 billion or US$5.4 billion for the Ministry of Defence in the supplemental budget. Together with additional spending for the Coast Guard and cybersecurity, the supplemental budget will bring Japan’s national security-related spending up to 2% of GDP for fiscal year 2025. Prime Minister Takaichi also announced her intention to revise the 2022 National Security Strategy, National Defense Strategy and Defence Buildup Plan within 2026 to further increase defence spending. She is also interested in studying a possible revision of Article 9 of the Japanese Constitution, establishing a national intelligence secretariat and loosening arms export restrictions to strengthen Japan’s defence industrial base. Japan is investing in a diverse range of new capabilities and is developing new legislation and doctrine. Japanese Maritime Self-Defence Force destroyers sailed through the Taiwan Strait in Februaryand June 2025 for the second and third time. Japan also established a Joint Operations Command in March 2025. Japan’s willingness to do more for itself reflects heightened abandonment concerns and responsiveness to US burden-sharing pressure.

At the same time, Japan remains committed to the US-Japan alliance: at the 28 October 2025 Takaichi-Trump summit, ‘the two leaders concurred to advance a broad range of security cooperation in order to further enhance the deterrence and response capabilities of the Japan-US alliance’. At the 8 February 2025 Ishiba-Trump summit, Japan received confirmation that the US would defend Japan, including the Senkaku/Diaoyu islands and through nuclear deterrence. In addition, Japan supports upgrading USFJ to a joint force headquarters, boosting US-Japan defence industrial cooperation and continuing training with the US to defend Japan’s remote islands and Taiwan. Prime Minister Takaichi’s intention to review the three non-nuclear principles has also drawn a lot of attention, though for now she upholds the government’s traditional stance. She is not interested in Japan producing or possessing nuclear weapons, but worries that the prohibition on the introduction of nuclear weapons weakens US extended deterrence. Japan’s willingness to continue working closely with the US is a reflection of low entrapment concerns.

Japan has been hesitant to diverge from US foreign policy towards adversaries. Even Prime Minister Takaichi’s overtures to North Korea are following President Trump’s own overtures to Kim. This is most apparent in response to US policy on Russia: after the infamous Oval Office meeting between Presidents Trump and Zelenskyy on 28 February 2025, then-Prime Minister Shigeru Ishiba emphasised that Japan has ‘no intention of taking sides’. Japan also followed the US lead in March when the Trump administration halted military aid – temporarily – by downgrading promises to “strengthen” support for Ukraine to “maintain” support for Ukraine. Yet beyond the rhetoric, Japan continues to support Ukraine: together with other partners, Japan’s commercial satellite firm provided geospatial intelligence that filled the void left by the suspension of US intelligence-sharing. Both premiers Ishiba and Takaichi participated in pro-Ukraine, “coalition of the willing” meetings, and Japan has been particularly active in demining efforts. In November 2025, Japan sent two Self-Defence Forces officers to Lithuania to conduct demining training for Ukrainian armed forces. Though it is a very small-scale, short-term deployment, it is still significant that the US military was not directly involved in the mission and it was not a US-Japan alliance initiative.

Japan is also continuing to deepen its security relationships with other US allies and partners – as it has been doing for the past decade or so – including Australia, the Philippines, Vietnam and India. Japan is also investing more in the G7, NATO, the Indo-Pacific Four and the “coalition of the willing” frameworks to deal with the Russia challenge. Most significantly, Japan under both premiers Ishiba and Takaichi has been trying to improve relations with South Korea. A Japanese Defence Minister visited South Korea for the first time in a decade in September 2025 to participate in the Seoul Defence Dialogue. Unfortunately, the ongoing Takeshima/Dokdo territorial dispute led to the cancellation of Japan-South Korea drills. It is also worth remembering, despite the current dispute over Prime Minister Takaichi’s Taiwan remarks, that in the initial months after President Trump was inaugurated, there was a flurry of activities to improve Sino-Japanese relations. In her meeting with President Xi, before the controversial remarks, Prime Minister Takaichi expressed her desire to ‘deepen their personal relationship’ and promote a ‘mutually beneficial relationship based on common strategic interests’. Japan is clearly diplomatically diversifying.

South Korea’s threat perceptions

For South Korea, the primary threat is always and foremost North Korea. President Lee Jae Myung began his presidency with a pro-engagement posture towards North Korea: seeking denuclearisation through inter-Korean dialogue. Unlike Japan, South Korea does not have a territorial dispute with China or Russia. It is possible that China and Russia can be partners to South Korea if they are willing to put pressure on North Korea. In his summit meeting with President Xi, President Lee asked him for his help in resuming talks with North Korea. President Lee prefers a “pragmatic” approach and has expressed a desire to have “amicable relations”, “trade with” and “cooperate with” Russia.

Compared to Japan, South Korea has been quieter with regard to both the ongoing Russian war on Ukraine and a potential Taiwan contingency. Before he was president, Lee criticised Yoon Suk Yeol’s contribution to Ukraine as amplifying tensions with North Korea and Russia. Though there is increasing US-South Korea rhetorical alignment on Taiwan, there is also still a lot of resistance to the idea of South Korea or even USFK being involved in a US-China war over Taiwan.

Given that North Korea is South Korea’s primary threat, US policy towards South Korea is indirectly US policy towards North Korea. Any US policy change that weakens the US deterrence of North Korea is a threat to South Korea. Indeed, policymakers in South Korea have resisted “strategic flexibility”, or the potential use of USFK to counter China. There are challenges with implementing “strategic flexibility”, as USFK is currently geared towards a land war and there is no guarantee that Pyongyang will not take advantage of a Taiwan contingency. However, it would be difficult for South Korea to remain neutral or uninvolved in a regional war and there are many ideas that deserve thorough debate about how the South Korean military, USFK and the US military can adapt for a “strategically flexible” posture. Another US policy that has implications for South Korea is how explicit US nuclear deterrence commitments should be. Washington prefers less explicit commitments to maintain its freedom of action, complicate North Korean response planning and reduce the risk of entrapment in a nuclear war; South Korea prefers more explicit commitments to prevent North Korean miscalculation and demonstrate US resolve.

South Korea’s response

In the lead-up to the Trump-Lee summit of 25 August 2025, South Korean government officials indicated their desire to avoid the discussion of strategic flexibility entirely. As President Lee said, ‘this [strategic flexibility] is not an issue that we can easily agree with’. While resisting strategic flexibility, South Korea has committed to increasing its defence spending. In early October 2025, President Lee announced that South Korea would increase the defence budget by 8.2% next year. During his 29 October 2025 summit meeting with President Trump, President Lee committed to increasing South Korea’s defence spending from its current 2.3% of GDP to 3.5% of GDP “as soon as possible”, which will likely be in 2035. The US also gave South Korea approval to build nuclear-powered attack submarines (SSNs). Given the delay between the presidential directive and the US-South Korea Factsheet, this was likely a case of President Trump getting ahead of the US interagency. However, the burden-sharing logic behind the political breakthrough is that South Korea can help the US maintain stability in sea lines of communication and strengthen South Korea’s deterrence against North Korea. Doing so will allow the US military to draw down from abroad and/or focus on deterring China. The resistance to strategic flexibility, increased defence spending and pursuit of SSNs indicates that South Korea has a high degree of abandonment concern; the increased defence spending and pursuit of SSNs are also in response to US pressure to burden-share.

South Korea demurs on “strategic flexibility” because it does not want USFK to be able to easily leave the Korean peninsula during a Taiwan contingency and because positioning the USFK to better assist in a Taiwan contingency is more likely to involve Seoul in a US-China war. One way that South Korea might be trying to deal with this is by pushing for the return of wartime operational control (OPCON) from the US. Trump’s first administration supported a conditions-based approach to wartime OPCON transfer and his current team also advocates for wartime OPCON transfer to ease the US defence footprint on the Korean peninsula. This is more a domestic political issue than a strategic issue, as reflected by the fact that it is the South Korean political leadership and not the military pushing for it. However, if wartime OPCON is returned to South Korea, it may be easier for South Korea to avoid being entrapped in a US-China war over Taiwan. Again, domestic political sensitivities over sovereignty are at least as relevant as the strategic calculus, but South Korea has heightened entrapment concerns given the Trump administration’s renewed (pending an updated NDS) focus on deterring China in the Taiwan Strait.

Since President Trump’s second administration was inaugurated, South Korea and the US have continued with their military training exercises, including Freedom Shield 25 in March and Ulchi Freedom Shield 25 in August 2025. Heat waves caused some exercises to be postponed and some have yet to be held. There is some speculation that the exercises are not being held because South Korea wants to send a conciliatory signal to North Korea. There is no speculation that there is hesitation on the US side for political reasons. In June 2018, President Trump gave Kim a surprise concession when he agreed to suspend US military exercises with South Korea. In March 2019, Key Resolve and Foal Eagle were cancelled and replaced by smaller exercises. In 2023, military exercises returned to the scale of the cancelled exercises with Freedom Shield and Warrior Shield FTX.

Though South Korea might prefer a more “pragmatic” approach to China and Russia, South Korea has not done anything that could be considered a dramatic policy divergence from the US. The most dramatic change in South Korea’s approach to China, Russia and North Korea is the proposal for military talks with North Korea. The purpose of the talks is to clarify the Military Demarcation Line and prevent possible clashes near the inter-Korean border. If North Korea accepts, then it will be the first inter-Korean military talks since 2018. Given President Trump’s outstanding invitation for a meeting with Kim and the attention paid to North Korea in the Korea-US Joint Fact Sheet, this should not be seen as a divergence from the US approach to North Korea.

South Korea is also seeking to deepen defence relationships with other US allies and partners. In South Korea’s case, these relationships are built on defence exports. South Korea is already the world’s 10^th largest arms exporter and President Lee wants South Korea to be the 4^th largest. Korean defence firms are particularly interested in exporting more to European countries. Though many observers are worried about the clash between a conservative leader in Japan – Prime Minister Takaichi – and a progressive leader in South Korea – President Lee – such fears seem to have been overblown. The ongoing dispute over the South Korean aircraft that flew around the disputed islands was not caused by the leaders and, so far, has not been escalated by the leaders. South Korea is also diplomatically diversifying, though perhaps in a way that is more tailored to its comparative advantage (i.e. defence exports). South Korea’s relationship-building with other US allies and partners will also be more constrained by the desire to maintain working relationships with China and Russia, especially with regard to the North Korea challenge.

Conclusion

In response to increased unpredictability in US policy towards China, Russia and North Korea, Japan is revamping its own defence capabilities and increasing security cooperation with the United States and other US allies and partners. Japan is also striving to maintain diplomatic alignment with Washington, especially on China and North Korea. Prime Minister Takaichi will try to follow in former Prime Minister Shinzo Abe’s footsteps of cultivating close personal ties with President Trump to mitigate the worst of his unpredictability. While President Trump’s unpredictability increases Japan’s insecurity, it accelerates preexisting trends within Japan towards Japan doing more for its own defence.

In broad contours, South Korea’s response is the same as Japan’s: investing in its own defence capabilities, trying to keep the United States committed to South Korea’s defence and broadening its security relationships with other partners. The greatest constraint that South Korea faces is North Korea. South Korea’s willingness to engage with China, Russia and other potential security partners will be a function of relative US, Chinese and Russian support for South Korea’s preferred North Korea policy.

One notable, threat-agnostic way that Japan and South Korea are trying to keep the US engaged in the region is through cooperation in shipbuilding. The US and Japan signed a Memorandum of Cooperation to expand shipbuilding capacity in both nations by aligning investment, procurement, workforce and technology initiatives. South Korea will invest US$150 billion in the shipbuilding sector in the US. South Korea and Japan are the second and third largest shipbuilders in the world, respectively, and possibilities for cooperation include joint ventures to construct and repair noncombatant vessels for the US Navy in the Indo-Pacific and Japanese and Korean investment in US shipyards. Though a trilateral, industrial shipbuilding alliance has been touted on strategic grounds, any such trilateral cooperation will likely be driven by President Trump’s interest in rebuilding US manufacturing capacity and clinching economic “deals” rather than countering China per se. This is just another reflection of how security and economic concerns are becoming increasingly intertwined – or transactional – in how US allies deal with a “Make America Great Again”-style US foreign policy.

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