Revival of Japan-South Korea Defense Cooperation: The Resumption of Maritime Joint Exercises After 9 Years Indicates a New Phase in Northeast Asian Security

On January 30, 2026, Japan's Minister of Defense Shinjiro Koizumi and South Korea's Minister of National Defense An Gyeong-bok met in Tokyo and agreed to resume joint maritime search and rescue training, which had been suspended since 2017. This decision signifies not only an improvement in bilateral relations but also a fundamental change in the security environment in Northeast Asia and a new phase of regional cooperation in response to it.

The most noteworthy aspect of this agreement is the inclusion of a framework for cooperation in the joint development and operation of artificial intelligence (AI) and unmanned weapon systems, in addition to traditional cooperation centered on humanitarian assistance. The two defense ministers confirmed their intention to enhance interoperability of defense capabilities through cooperation in these advanced technology fields and to strengthen deterrence against North Korea's nuclear and missile threats and China's maritime expansion.

The background to the suspension of Japan-South Korea defense cooperation includes complex political factors such as the December 2018 fire control radar incident, historical recognition issues, and territorial disputes over Takeshima (Korean name: Dokdo). Since the suspension of regular search and rescue training until 2017, exchanges at the practical level between the Japan Maritime Self-Defense Force and the South Korean Navy had also significantly decreased. However, the security environment in Northeast Asia has dramatically worsened over the past nine years.

On January 5, 2026, North Korea conducted a hypersonic missile launch test toward the Sea of Japan early in the new year. Kim Jong-un emphasized the necessity of nuclear deterrence due to the "recent geopolitical crisis" and indicated a continued stance of provocation. In his New Year address, Admiral Akira Saito, Chief of Staff of the Japan Maritime Self-Defense Force, pointed out that tensions in the Taiwan Strait and the South China Sea due to China's maritime expansion, Russia's invasion of Ukraine, and North Korea's ballistic missile launches continue to shake the international order, stating, "The role of the Japan Maritime Self-Defense Force is becoming increasingly important."

China's military rise is also a significant driving force behind the resumption of Japan-South Korea cooperation. The People's Liberation Army Navy of China has intensified its activities in the East China Sea and the Sea of Japan, with incursions into Japanese territorial waters and passage through contiguous zones around Japan's southwestern islands becoming routine. Furthermore, threats in cyberspace and outer space are increasing, making cooperation in new operational domains essential in addition to traditional maritime and aerial domains.

Japan's defense capability development is also progressing rapidly. The Ministry of Defense announced plans to deploy a new type of land-based anti-ship missile, extended to a range of approximately 1,000 kilometers, to the 5th Anti-Ship Missile Regiment stationed at the Ground Self-Defense Force's Kengun Base in Kumamoto Prefecture between fiscal years 2025 and 2026. Additionally, the launch of new attack submarines and mass production of unmanned systems are underway, with these capabilities forming the core of deterrence in island defense. Japan's defense budget for fiscal year 2026 is set to reach a record high, including the development of amphibious operation response capabilities using unmanned systems under the SHIELD (Synchronized, Hybrid, Integrated and Enhanced Littoral Defense) concept, which integrates multiple Self-Defense Force branches.

South Korea is also promoting its own defense capability enhancement. In recent years, the South Korean military has been expanding its missile defense systems, constructing additional Aegis destroyers, and advancing its own aircraft carrier construction plans, with these capabilities being complementary to Japan's defense posture. In particular, both countries possess technological strengths in the development of AI-based information collection and analysis systems and unmanned surface vessels, and synergistic effects from joint research and development are expected.

This agreement also holds significant meaning for the framework of trilateral security cooperation among Japan, the U.S., and South Korea. The United States has long sought to strengthen defense cooperation between its allies Japan and South Korea, but historical issues between the two countries have posed barriers. The resumption of training and the agreement on cooperation in AI and unmanned weapon systems are seen as important progress in strengthening the trilateral cooperation framework in the U.S. Indo-Pacific strategy.

However, challenges remain. There is still deep-rooted mutual distrust in the domestic public opinion of both Japan and South Korea, and it is uncertain whether this agreement will be maintained if historical or territorial issues reignite. North Korea has criticized Japan's defense capability enhancement as a "revival of past militarism," and the strengthening of Japan-South Korea cooperation may become a new target for criticism.

Looking ahead, it is expected that Japan and South Korea will gradually expand the scope of their cooperation. They plan to conduct search and rescue training within 2026 and, based on the results, may develop joint training in more advanced operational areas such as anti-submarine warfare and air combat in the future. Additionally, in the joint development of AI and unmanned weapon systems, discussions at the practical level regarding the sharing of technical information and ensuring interoperability are likely to accelerate.

As the security environment in Northeast Asia becomes increasingly severe, the resumption of practical defense cooperation between Japan and South Korea, overcoming historical conflicts, is an important step for regional stability. However, for this cooperation to be truly sustainable, it is essential to enhance military transparency and establish crisis management mechanisms alongside building trust at the political level. The resumption of joint maritime training after nine years is merely the starting point.