A Historic Turning Point in Japan-South Korea Defense Cooperation: Resumption of Joint Exercises After 9 Years and Deepening Collaboration Among the Three Northeast Asian Countries

At the beginning of 2026, the security environment in Northeast Asia is reaching a critical turning point. Notably, on January 30, Japan's Minister of Defense Shinjiro Koizumi and South Korea's Minister of National Defense Lee Jong-sup met at the Yokosuka Maritime Self-Defense Force base and agreed to resume joint search and rescue training between Japan and South Korea, which had been suspended since 2017. This decision symbolizes a pragmatic shift prioritizing practical-level cooperation in the defense relations of both countries, which had been strained due to historical and territorial issues, in the face of common security challenges such as China's maritime expansion, North Korea's nuclear and missile threats, and Russia's military activities.

According to a joint statement released by the defense authorities of both countries, this agreement encompasses not only the resumption of joint training for humanitarian purposes but also a comprehensive framework to "revitalize personnel exchanges and unit exchanges to build mutual understanding and trust between the South Korean military and the Self-Defense Forces." Furthermore, it is significant that both ministers agreed to strengthen cooperation in the development of artificial intelligence and unmanned weapon systems. Cooperation in this area is based on pragmatic judgments of technological complementarity and adaptation to future operational environments, transcending traditional political constraints. In particular, South Korea is rapidly advancing the development of AI-driven drones, which General Secretary Kim Jong-un has designated as a "top priority," with unmanned aerial vehicles showcased at the Air Force memorial ceremony in November and entering mass production for FPV drones and battlefield drones. Meanwhile, Japan's budget request for fiscal year 2026 includes procurement of a multi-service integrated drone system based on the "Synchronized, Hybrid, Integrated and Enhanced Littoral Defense (SHIELD) concept," directly linking the technological cooperation of both countries to mutual capability enhancement.

The restart of Japan-South Korea defense cooperation is positioned within a larger strategic context of strengthening trilateral security cooperation that includes the United States. The Japan-U.S. joint exercise "Iron Fist," conducted from February to March 2026, involved approximately 2,000 personnel from the Japan Ground Self-Defense Force and about 3,000 from the U.S. military, showcasing large-scale defense training. This exercise demonstrates that the Japan-U.S. alliance is evolving from traditional abstract security cooperation to the integration of concrete operational capabilities. Notably, the exercise emphasizes island defense and counter-landing operations, clearly keeping in mind the activities of the People's Liberation Army of China in the East China Sea and the Taiwan Strait.

Additionally, from February 23 to 27, the "Fleet Synthetic Training Joint: FST-J 26-71," conducted at the U.S. Forces Japan headquarters in Yokosuka and regional command centers, provided an important opportunity to verify the integrated operational capabilities of Japan and the U.S. naval and air forces. Such simulation training can be conducted more frequently than live-fire exercises and allows for repeated testing of complex scenarios, making it extremely effective for improving Japan-U.S. interoperability. The training involved the Japan Maritime Self-Defense Force and U.S. Navy surface and air units, executing integrated scenarios such as maritime surveillance, anti-submarine warfare, and air defense operations.

In terms of air power, Japan-U.S. cooperation has been significantly strengthened. On February 16 and 18, a joint Japan-U.S. air exercise involving four U.S. Air Force bombers was conducted over the Sea of Japan and the East China Sea. This exercise also included Japan's Air Self-Defense Force F-2A fighters, validating advanced operational techniques such as long-range strike capabilities, fighter coordination, and aerial refueling support. The deployment of U.S. strategic bombers sends a clear message regarding the U.S. power projection capabilities in the Indo-Pacific region and its commitment to Japan's defense. In his New Year remarks for 2026, Japan Maritime Self-Defense Force Chief of Staff Akira Saito pointed out that "China's maritime expansion and tensions in the Taiwan Strait and South China Sea, Russia's invasion of Ukraine, and North Korea's ballistic missile launches continue to shake the international order, and threats in cyberspace and outer space are also increasing," emphasizing that "in light of this reality, the role of the Japan Maritime Self-Defense Force is becoming increasingly important."

In fact, the Japan Maritime Self-Defense Force is constantly tracking vessels of the People's Liberation Army Navy and the Russian Navy as they navigate near Japan, enhancing monitoring activities in strategic straits, including the Tsushima Strait. According to publicly available data from the Ministry of Defense, activities of Chinese naval vessels in waters around Japan increased in 2025 compared to the previous year, particularly with the normalization of the deployment of aircraft carrier strike groups centered around the aircraft carriers "Liaoning" and "Shandong" into the Pacific. In response to this situation, Japan has included new procurement of anti-landing craft missiles in its fiscal year 2026 budget, urgently strengthening its island defense capabilities.

At the political level, Japan's security policy is also undergoing a significant turning point. Prime Minister Sanae Takaichi has expressed the intention to advance the revision of the national security strategy, defense strategy, and defense capability development plan to be implemented within 2026, ahead of the original schedule. According to an analysis by the International Crisis Group, this revision suggests that "considering the maritime and aerial nature of the threats Japan faces, the Japan Ground Self-Defense Force's dominance in troop composition should be corrected." Currently, the distribution of personnel in the Self-Defense Forces reflects a legacy of Cold War-era emphasis on northern defense, with the Japan Ground Self-Defense Force having the largest number of personnel. However, to respond to China's maritime expansion and North Korea's missile threats, it is necessary to further enhance resource allocation to the Japan Maritime Self-Defense Force and the Japan Air Self-Defense Force.

The institutional foundation of the Japan-U.S. alliance is also at a point for renegotiation. The Host Nation Support agreement between Japan and the U.S. came into effect in 2022 and is set to expire in fiscal year 2026, so negotiations are currently underway. Japan's burden under this agreement is considered one of the most generous host nation supports in the world, covering operational costs of U.S. military bases in Japan, salaries for Japanese employees, training expenses, and more. The new agreement is expected to focus on the impacts of inflation and the increased costs of facility improvements necessary for enhancing U.S. military operational capabilities.

On the other hand, North Korea conducted its first ballistic missile launch of the new year on January 5, 2026, firing a missile that flew approximately 900 kilometers toward the Sea of Japan. According to the South Korean Joint Chiefs of Staff, the detailed specifications of the missile are being comprehensively analyzed among the U.S., South Korea, and Japan, stating that "under a solid South Korea-U.S. combined defense posture, our military maintains a state of vigilance against various activities from North Korea and possesses the capability and readiness to respond overwhelmingly to any provocation." This launch indicates that North Korea continues its missile development despite international sanctions and diplomatic isolation, highlighting the necessity for strengthened cooperation among Japan, South Korea, and the U.S.

At a broader regional level, Japan is also strengthening military cooperation with New Zealand. In 2024, New Zealand Prime Minister Chris Hipkins and then-Prime Minister Fumio Kishida signed an information-sharing agreement and agreed to increase joint activities and exercises in the field of maritime security. As part of this, the New Zealand Navy's replenishment ship "Aotearoa" was deployed as part of the North Korea containment strategy, and personnel in the demilitarized zone were significantly increased from 12 to 53. This indicates that Japan's security cooperation is deepening beyond traditional partners like the U.S. and South Korea, engaging with democratic nations across the Indo-Pacific region.

Considering this series of developments, it can be said that defense cooperation in Northeast Asia in 2026 has entered a qualitatively new phase. First, in Japan-South Korea relations, there is a growing tendency for security pragmatism to take precedence over emotional factors related to historical issues. Second, the Japan-U.S. alliance is evolving from traditional abstract security guarantees to the construction of concrete integrated operational capabilities. Third, trilateral cooperation (Japan-U.S.-South Korea) is being institutionalized, forming a permanent framework for information sharing, joint exercises, and technological cooperation. Fourth, Japan's own defense capability development is shifting from passive base provision to the active construction of denial defense.

However, at the same time, these movements also carry the potential to heighten regional tensions. China has criticized the strengthening of Japan-U.S.-South Korea military cooperation as "new Cold War thinking," and Russia is deepening its military cooperation with North Korea. The National Defense Authorization Act for fiscal year 2026 includes provisions calling for inter-agency cooperation to disrupt the activities of the "dark quartet" of China, Russia, North Korea, and Iran, and U.S. strategic documents clearly position countering China as the top priority. Notably, the national defense strategy for 2026 orders U.S. forces to "build, deploy, and maintain a strong denial defense along the first island chain," viewing the island chain from Japan to Taiwan, the Philippines, and Indonesia as a geographical fortress to prevent China's military expansion.

Looking ahead, Japan's defense policy makers need to enhance their analytical capabilities to align limited resources with strategic objectives. Considering threats from the maritime and aerial domains, it is essential to correct the current troop composition that is biased toward the Japan Ground Self-Defense Force and prioritize investment in the Japan Maritime Self-Defense Force and the Japan Air Self-Defense Force. Additionally, regarding Japan-South Korea cooperation, while political constraints still exist, the accumulation of practical-level cooperation can lead to trust-building and potentially open the path to more advanced operational cooperation in the future. In relations with the U.S., alongside negotiations on host nation support, further enhancement of integrated operational capabilities, integration of missile defense systems, and strengthening cooperation in cyberspace and outer space will be challenges.

In conclusion, the defense trends in Northeast Asia in 2026 reflect the reality that Japan, South Korea, and the U.S. must deepen security cooperation, even setting aside historical conflicts, in the face of the dual threats posed by China's military rise and North Korea's provocations. The resumption of Japan-South Korea joint training after nine years is a symbolic event that underscores the severity of the security environment in this region and the necessity for practical cooperation, serving as a litmus test for future trilateral cooperation. At the same time, it is important to recognize that this strengthening of cooperation may accelerate regional arms races and increase the risk of accidental conflicts. Japan's defense policy is required to adapt to the changing strategic environment while carefully balancing the enhancement of deterrence and the de-escalation of tensions through dialogue.