A New Horizon for Japan-South Korea Defense Cooperation: The Resumption of Naval Exercises and the Deployment of Tomahawks Indicate a Turning Point in Northeast Asian Security

In late January 2026, the Japan-South Korea defense ministerial meeting held in Yokosuka, Kanagawa Prefecture, brought about a significant turning point in the security environment of the Northeast Asia region. The defense leaders of both countries agreed to resume naval exercises that had been temporarily suspended and clearly articulated the strengthening of their joint response capabilities against the increasing threats from China and North Korea. This move is not merely a revival of bilateral cooperation but is noted as a symbolic event suggesting the reorganization of the security network among Japan, the United States, and South Korea.

At the same time, Japan took an important step towards a qualitative transformation of the Self-Defense Forces' strike capabilities. The Japan Maritime Self-Defense Force's Aegis destroyer "Chokai" (DDG-176) is expected to complete the installation of Tomahawk cruise missiles by the end of 2026, thereby acquiring its first stand-off defense capability at sea. The introduction of this weapon system, with a range of approximately 1,600 kilometers, signifies a historic shift in Japan's security policy, which has traditionally adhered to a principle of exclusively defensive posture.

The backdrop of Japan-South Korea defense cooperation lies in the common strategic challenges faced by both countries. Air Self-Defense Force Chief of Staff Takehiro Morita pointed out in his New Year address in 2026 that activities by the Chinese Air Force around Japan are "becoming increasingly serious and complex year by year." The increase in activities by Chinese aircraft carrier-based aircraft, helicopters from the Coast Guard, and drones indicates a rapid change in the balance of power in the East China Sea. Similarly, the frequency of ballistic missile launches by North Korea is also rising, compelling both countries to address multi-layered threats.

In this context, the significance of Japan and South Korea agreeing to resume naval exercises is substantial. The 2015 Peace and Security Legislation allowed Japan to exercise limited collective self-defense, establishing a legal framework for joint operations with the U.S. military in defense of South Korea and Taiwan. However, due to historical recognition issues and political conflicts, substantial military cooperation had stagnated for many years. This recent agreement signifies a shift in both countries towards prioritizing practical security benefits.

Particularly noteworthy is the close linkage of this move with U.S. regional strategy. The United States has announced plans to conduct over 500 military exercises with the Philippines in 2026, clearly demonstrating its stance against Chinese naval activities in the South China Sea. The strengthening of Japan-South Korea cooperation is positioned along the northern extension of this U.S.-led alliance network. In fact, the defense cooperation among Japan, the U.S., and South Korea reflects a new geopolitical reality where the East China Sea and South China Sea can no longer be treated as separate operational domains.

The strategic impact of the Tomahawk deployment cannot be overlooked. The Japan Maritime Self-Defense Force has been advancing the acquisition of long-range precision strike capabilities alongside the conversion of Izumo-class destroyers into aircraft carriers. The installation of Tomahawks on "Chokai" represents the first phase of this long-term capability-building plan. Sea-launched Tomahawks have higher survivability compared to land-based missile systems and can ensure political and diplomatic flexibility. In countering long-range missile systems from China and Russia, Japan is transitioning its position from being "on the receiving end of the first strike" to "having retaliatory capabilities."

However, this policy shift also has aspects that require careful consideration. The Japanese government has set a goal to raise defense spending to 2% of GDP by March 2026 and has decided on the highest defense budget in history. This budget increase includes costs for acquiring Tomahawks, upgrading Aegis ships, and organizing elite units by integrating the Japan Ground Self-Defense Force's Special Operations Group and Central Readiness Regiment. Financial sustainability and domestic discussions surrounding the interpretation of Article 9 of the Constitution will continue to be ongoing challenges.

Other countries in the region are also closely watching this change. Hu Bo, a Chinese expert on maritime policy, recently argued in an essay that "China has sufficient capability to manage and control the situations in the East China Sea and South China Sea and can patiently maintain the status quo." This reflects China's confidence in strengthening military cooperation among Japan, the U.S., and South Korea, but it also foreshadows an intensification of the arms race in the region. Vietnam's plan to complete infrastructure development in the Spratly Islands by 2026 and to station mixed units of civilians and naval infantry will further heighten tensions in the South China Sea.

Additionally, 2026 is also the year for renegotiating the Special Measures Agreement (SMA) regarding the burden of U.S. military stationing costs in Japan. In past negotiations, the U.S. side has repeatedly demanded an increase in burden-sharing, but with the deployment of Tomahawks and the enhancement of Aegis capabilities, Japan's defense contributions have qualitatively expanded. This point could serve as a bargaining chip for Japan in future negotiations.

The security environment in Northeast Asia is facing "unprecedented challenges." This expression, frequently reiterated by Japan's defense leaders, is not mere rhetoric but reflects a fundamental transformation of the regional order that has been maintained for over 70 years since the end of World War II. The resumption of Japan-South Korea naval exercises and the deployment of Tomahawks should be understood as adaptive measures to this new reality. Both countries are seeking to enhance the reliability of their deterrent capabilities by deepening mutual defense cooperation while maintaining their alliance with the United States.

However, merely strengthening military deterrence will not achieve regional stability. There are complex issues intertwined with territorial disputes, historical recognition, and economic interdependence among China, South Korea, and North Korea. Japan needs to maintain diplomatic dialogue channels alongside the development of its defense capabilities and establish crisis management mechanisms to avoid unforeseen circumstances. The current difficulties in negotiations for a Code of Conduct (COC) between China and ASEAN countries in the South China Sea illustrate the challenges of building such multilateral frameworks.

Northeast Asia in 2026 is at a turning point where the modernization of military power, the reorganization of alliances, and the redefinition of regional order are occurring simultaneously. The progress of Japan-South Korea defense cooperation and the deployment of Tomahawks are important chapters in this larger trend and will serve as a litmus test for regional dynamics in the coming years. The international community is watching closely whether both countries can deepen practical cooperation beyond historical conflicts and whether changes in military balance can lead to the establishment of a peaceful regional order.