Changes in the Security Environment of East Asia: The Japan-U.S. Alliance and Naval Strategy in Northeast Asia (2026 Edition)

The security environment in East Asia is undergoing complex changes in 2026. Japan has approved its largest defense budget in history, and Japan and South Korea are resuming joint exercises for the first time in ten years, while China's military activities are becoming more active, and the threat from North Korea is transitioning to a new phase. This change is not merely a temporary phenomenon but fundamentally redefines the region's long-term security structure.

On December 26, 2025, the Japanese government approved a historic defense budget proposal of 9.4 trillion yen (approximately 58 billion dollars). This amount approaches 2% of Japan's GDP and represents a significant step toward achieving the goal of a defense spending-to-GDP ratio of 2%, which has been a target since former Prime Minister Shinzo Abe. More than 10% of the budget is allocated to enhancing long-range strike capabilities, which is a concrete measure to counter China's military threats. Notably, there are plans to convert two Izumo-class destroyers (JS Izumo and JS Kaga) of the Japan Maritime Self-Defense Force into aircraft carriers capable of carrying F-35B jets. The budget allocates 182.4 million dollars for this purpose, and the conversion work is expected to intensify within 2026.

The improvement in Japan-South Korea relations is also evident in the field of military cooperation. The defense ministers of both countries met in Yokosuka on January 30, 2026, and agreed to resume joint maritime search and rescue exercises that had been suspended since 2017. While this exercise has a humanitarian nature, it is set against the backdrop of the deteriorating security environment in the region, including the missile threat from North Korea. According to announcements from South Korea's Ministry of National Defense, the two countries plan to expand not only maritime search and rescue exercises but also the sharing of maritime surveillance information. This improvement in cooperation is extremely important for strengthening the U.S.-led regional security framework.

China's military activities have also become more active since the beginning of 2026. From January 9 to 12, a large-scale gathering of Chinese naval militia was observed in the East China Sea. This movement is analyzed as indicating dissatisfaction towards Japan. Furthermore, large-scale military exercises by the Chinese military were conducted around Taiwan from December 29 to 30, 2025. These exercises simulated a blockade of Taiwan and are referred to as "Mission Justice-2025." The Japanese government has condemned these exercises as "actions that increase tensions across the Taiwan Strait," clearly indicating an intention to deter military cooperation between Japan and the U.S.

Technological advancements in the security environment of Northeast Asia are also remarkable. On April 1, 2026, Japan's Ground Self-Defense Force announced the deployment of its first long-range anti-ship missile, the "Type 25 series." This missile is capable of reaching speeds of Mach 5, with a range of 500 to 900 km. South Korea also revealed the existence of air-to-ship hypersonic missiles on February 19, indicating that the country's hypersonic missile program is transitioning from mere technological demonstrations to practical operational systems. North Korea also launched a hypersonic missile toward the East Sea on January 5, 2026, intensifying military competition in the region.

Strengthening the Japan-U.S. alliance is also a key focus. The U.S. defense strategy for 2026 clearly outlines a reorganization of deterrence in the Indo-Pacific. This strategy proposes an increase in naval vessels in Guam/Pacific, the establishment of missile bases and radar on key islands, and the expansion of rotational forces of the Army and Marine Corps to Japan, South Korea, and Australia. Particularly important is the role of Japan and South Korea in the second pillar of AUKUS. Although neither country is a member of AUKUS, they are seen as optimal partners in the "Trilateral Defense Cooperation Framework" of the second pillar, with expectations for joint development of advanced military technologies.

Maritime security in the East China Sea and the East Sea is extremely important from the perspective of ensuring international legal order and freedom of navigation. Japan emphasizes compliance with maritime legal norms and is compelled to respond to the activities of the Chinese naval militia. Given the situation in 2026, military tensions in this maritime area are likely to persist in the long term. The Japanese government needs to take concrete measures to protect its maritime interests while strengthening cooperation with regional countries based on the concept of a "free and open Indo-Pacific."

The security environment in Northeast Asia is in a complex state, intertwined with technological innovation, reorganization of alliances, and changes in the strategic goals of various countries. The strengthening of Japan's defense capabilities, the improvement of Japan-South Korea relations, and the deepening of trilateral cooperation among the U.S., Japan, and South Korea will serve as the foundation for long-term peace and stability in the region. However, further efforts are needed to address fundamental challenges such as China's military threats and North Korea's provocations. The trends in 2026 will be a crucial turning point indicating the direction of future security in East Asia.

Countries in the region should aim for problem-solving through dialogue and cooperation rather than military confrontation. Upholding the principles of freedom of navigation and compliance with international law, and building mutual trust, are believed to lead to ultimate regional stability. Changes in Japan's defense policy will have a significant impact on the security structure of this region, but it is also important to maintain a commitment to constructive international cooperation.