From Historical Rivals to Security Partners: Can Seoul and Tokyo Sustain Their Defence Reset?

South Korea and Japan’s renewed defence cooperation signals a pragmatic shift toward shared security priorities amid North Korea’s threats and Indo-Pacific tensions. The move may strengthen deterrence, policy coordination and economic-security ties, but historical disputes remain the biggest risk to long-term progress.

South Korea and Japan's decision to revive joint search-and-rescue exercises and reaffirm their commitment to the denuclearisation of the Korean Peninsula signals more than the restoration of a military activity, it reflects a strategic shift in Northeast Asia where shared security concerns are increasingly taking precedence over decades of historical and political disagreements. The meeting between South Korean Defence Minister Ahn Gyu-back and Japanese Defence Minister Shinjiro Koizumi comes as North Korea continues advancing its nuclear and missile capabilities, while geopolitical competition in the Indo-Pacific intensifies. Encouraged by the United States, Seoul and Tokyo are gradually rebuilding defence cooperation, recognising that regional stability increasingly depends on closer coordination among like-minded partners.

From Historical Rivals to Strategic Security Partners

The latest agreement represents another milestone in repairing a relationship that has frequently been strained by unresolved issues stemming from Japan's colonial rule of the Korean Peninsula, wartime labour disputes, and territorial disagreements. These disputes have periodically disrupted diplomatic engagement and even threatened security cooperation, as demonstrated in 2019 when South Korea considered ending its intelligence-sharing agreement with Japan following trade tensions.

Today's security environment, however, is fundamentally different. North Korea's continued missile launches and nuclear development have increased the urgency for both governments to cooperate more closely. Reviving joint search-and-rescue exercises is therefore not simply a humanitarian initiative but also a confidence-building measure that improves military interoperability, communication channels, and operational readiness during regional emergencies.

The agreement also aligns with the United States' long-standing objective of strengthening trilateral security cooperation among Washington, Seoul, and Tokyo to enhance deterrence and maintain stability in Northeast Asia.

How South Korea and Japan Stand to Benefit

For South Korea, deeper defence cooperation with Japan enhances national security by improving intelligence coordination, maritime surveillance, and crisis-response capabilities. Working more closely with Japan allows Seoul to respond more effectively to regional security threats while reducing operational gaps during emergencies involving North Korea or broader regional contingencies. Improved diplomatic relations may also create opportunities for broader collaboration in advanced technologies, semiconductor supply chains, cybersecurity, and clean energy sectors, which are increasingly viewed as components of national security.

Japan also gains significant strategic advantages. The renewed partnership strengthens Tokyo's efforts to play a more active regional security role while complementing its expanded defence strategy. Better coordination with South Korea improves Japan's ability to monitor regional developments, conduct joint humanitarian operations, and strengthen maritime security. At the same time, stable political relations with one of Asia's largest economies can reduce geopolitical uncertainty affecting trade, technology investment, and industrial cooperation.

However, neither country is free from political constraints. Public opinion remains sensitive to historical grievances, meaning future governments must continue balancing domestic political pressures with strategic security priorities.

Why Policymakers and Stakeholders Should Pay Close Attention

For policymakers, the renewed defence partnership demonstrates that practical cooperation can advance even when historical disputes remain unresolved. Rather than waiting for comprehensive political reconciliation, both governments are prioritising shared security interests through incremental confidence-building measures. This approach could become a model for managing complex bilateral relationships elsewhere in the Indo-Pacific.

The agreement also reinforces the importance of institutionalising defence cooperation. Regular military exercises, intelligence exchanges, cybersecurity coordination, and defence dialogues reduce dependence on political leadership changes and create more resilient security partnerships over time.

For businesses and private-sector stakeholders, improved South Korea-Japan relations reduce geopolitical uncertainty in sectors where both countries play global leadership roles, including semiconductors, electronics, batteries, artificial intelligence, and critical technologies. More stable political relations can strengthen supply-chain resilience and encourage greater cross-border investment, research collaboration, and industrial partnerships.

Regional stakeholders, including ASEAN members, Australia, and other Indo-Pacific partners, may view stronger Seoul-Tokyo cooperation as contributing to a more coordinated regional security framework. Conversely, China and North Korea may perceive the closer defence partnership as reinforcing a broader U.S.-aligned strategic architecture, potentially adding further complexity to regional diplomacy.

The Real Test Lies Beyond Military Exercises

While restarting joint search-and-rescue drills is an important confidence-building measure, the longer-term success of South Korea-Japan relations will depend on whether both governments can prevent historical disputes from repeatedly undermining strategic cooperation.

Future progress will likely be measured through expanded intelligence sharing, greater defence industrial collaboration, coordinated cybersecurity initiatives, and closer cooperation in emerging technologies. Continued diplomatic engagement on energy security, supply-chain resilience, and economic security would further demonstrate that bilateral relations are becoming broader and more institutionalised.

At the same time, unresolved historical issues remain politically sensitive and could resurface during elections or diplomatic disputes. Territorial disagreements and wartime compensation issues continue to carry strong domestic political significance in both countries, meaning policymakers will need sustained political commitment to maintain the current momentum.

Ultimately, this renewed defence cooperation reflects a broader shift in Northeast Asia's strategic landscape. Faced with an increasingly uncertain regional security environment, South Korea and Japan appear to be placing greater emphasis on shared strategic interests than on longstanding historical divisions. Whether this evolves into a durable strategic partnership will depend not only on military cooperation but also on both governments' ability to build lasting political trust, deepen economic collaboration, and institutionalise mechanisms that allow cooperation to continue even when historical disagreements inevitably re-emerge.