With Donald Trump’s return to the presidency now a reality, the Indo-Pacific faces an era of heightened uncertainty driven by North Korea’s growing military capabilities and China’s expanding regional influence. In this environment, trilateral security cooperation between the United States, South Korea and Japan is paramount.

However, Trump’s typical approach to alliances—characterised by unpredictability and transactional diplomacy—poses a challenge to this partnership. A recalibration of policies and priorities will be necessary to ensure that trilateral cooperation is effective and sustainable.

The Indo-Pacific security environment has evolved since Trump’s first term. North Korea has accelerated its nuclear and missile programs, with increasingly sophisticated weapons systems capable of threatening regional and global stability. China’s assertiveness has intensified, manifesting as aggressive actions in the South and East China Seas, economic coercion and expanded military presence.

North Korea presents the most immediate and existential challenge. Trump’s initial approach to Pyongyang, centred on high-profile summits with Kim Jong-un, produced no lasting denuclearisation outcomes. In Trump’s second term, shifts between direct engagement and heightened pressure could create an unpredictable policy environment.

Trilateral security cooperation could be a stabilising mechanism, enabling the three countries to align their deterrence strategies. Integrated missile defence systems, intelligence-sharing networks and joint military exercises are essential tools to counter North Korea’s provocations. Policymakers must also focus on closing operational gaps, such as improving interoperability between the US’s THAAD missile defence system and South Korea and Japan’s Aegis-based defences, to enhance collective security.

China’s regional ambitions require similarly urgent attention. Trump’s return is likely to intensify US-China competition, with a focus on economic decoupling, technological dominance and countering Beijing’s maritime expansion. Cooperation between South Korea, Japan and the US could reinforce freedom of navigation operations in contested waters, secure critical supply chains and strengthen cybersecurity defence. However, Trump’s past insistence on burden-sharing—such as his calls for increased financial repayment for US troop presence—could complicate this dynamic, particularly if allies perceive these demands as undermining mutual trust or commitment to shared objectives.

Historical tensions between South Korea and Japan could also obstruct effective trilateral collaboration. Despite recent steps toward reconciliation, unresolved issues related to historical grievances and territorial disputes continue to strain bilateral relations. The new Trump administration must act as a mediator to prevent these tensions from undermining collective efforts. This will require consistent diplomatic leadership, which was often lacking in Trump’s first term. By institutionalising mechanisms for dialogue and cooperation, such as trilateral defence ministerial meetings and joint crisis planning, the US can help ensure that Seoul and Tokyo remain focused on shared strategic priorities.

Policy implications for a second Trump presidency extend beyond traditional security measures. The evolving nature of threats, including economic security, cyber warfare and technological competition, demands a more comprehensive approach to trilateral cooperation.

Policymakers should prioritise joint investments in critical technologies, such as semiconductor manufacturing and artificial intelligence, to reduce dependency on China and bolster resilience. Expanding cooperation in space-based surveillance and defence systems would further enhance the alliance’s ability to respond to emerging threats. Additionally, public diplomacy is needed to foster greater mutual understanding and support for trilateral cooperation among the populations of all three nations, countering domestic scepticism fuelled by nationalism.

To ensure the long-term viability of trilateral security cooperation, the Trump administration must adopt a more strategic and less transactional approach to alliances. This includes reaffirming commitments to collective defence and providing clear and consistent communication.

Ultimately, Trump’s return to office presents both challenges and opportunities for trilateral security cooperation. While his leadership style and unpredictability may strain alliances, the strategic necessity of collaboration between the US, South Korea and Japan remains undeniable. By addressing operational gaps and expanding the scope of cooperation to include emerging security domains, the trilateral alliance can serve as a cornerstone of Indo-Pacific stability.

In a region defined by rapid change and intensifying competition, the success of this partnership will be a determinant of future peace and security.