Japan and South Korea played prominent roles in this year’s U.S.-led Rim of the Pacific (Rimpac) military exercises, prompting renewed discussion about the potential for closer defence cooperation between the two neighbors despite longstanding historical tensions. The exercises, held in the waters off Hawaii from June 24 to July 31, marked the largest multinational maritime drill in the world, involving 30 countries, over 30,000 personnel, and a vast array of naval and air assets.

South Korean Rear Admiral In-ho Kim served as commander of the combined forces maritime component during Rimpac, while Japanese Rear Admiral Takuo Kobayashi took on the role of vice-commander of the combined task force. U.S. Vice-Admiral Jeffrey Jablon, who commanded the overall combined task force, highlighted the rapid integration of the multinational force, noting that key leadership roles held by officers from Chile, South Korea, Canada, and Japan enabled operations to begin cohesively from the outset.

Despite ongoing disputes rooted in Japan’s colonization of the Korean Peninsula from 1910 to 1945, the high-profile positions occupied by senior officers from both countries have been viewed by analysts as a sign of potential progress toward enhanced operational collaboration. William Cho, senior fellow and deputy director of the Japan chair at the Hudson Institute, emphasized that the leadership roles held by South Korea and Japan at Rimpac could serve as a foundation for deeper military and naval cooperation.

Yuko Nakano, associate director of the US-Japan strategic leadership program at the Center for Strategic and International Studies, pointed to groundwork laid by former South Korean President Yoon Suk-yeol and former Japanese Prime Minister Fumio Kishida, including their participation in the 2023 trilateral summit at Camp David hosted by the Biden administration. While political leadership in the U.S., Japan, and South Korea has changed since then, ongoing security dialogue remains stable and mature, Cho said, with trilateral cooperation aligning with broader U.S. alliance strategies.

Concerns over China’s expanding military capabilities and North Korea’s persistent nuclear and missile threats continue to drive security cooperation among the three countries. However, South Korea and Japan’s direct defense ties remain complicated by unresolved historical disputes, sovereignty issues, and divergent policy priorities. For example, while Japan has shown interest in pursuing the Acquisition and Cross-Servicing Agreement—a bilateral military logistics pact to facilitate the exchange of supplies and support—South Korea has approached such initiatives cautiously, citing the sensitivity of direct bilateral cooperation.

South Korean President Yoon Suk-yeol has emphasized that Tokyo must address historical grievances before formalizing deeper military partnerships with Seoul. Nevertheless, U.S. involvement in trilateral security arrangements is seen as a factor that may make such cooperation more acceptable to domestic audiences in both countries.

Experts suggest that while defense cooperation between Japan and South Korea may continue to be structured around the U.S.-led “hub-and-spoke” alliance model, there is still significant work to be done to enhance practical cooperation. Despite difficulties, both nations are expected to maintain at least a basic level of defense collaboration, driven by shared concerns about North Korea’s belligerence, which has included numerous missile tests near their territorial waters.

However, some analysts caution that coordination on North Korea-related threats may not extend as far as other areas covered by Rimpac. The exercises focused primarily on sophisticated maritime threats, which differ from challenges posed by North Korea’s missile program. Moreover, Japan and South Korea’s approaches to regional contingencies—such as those involving Taiwan or the South China Sea—may diverge due to differing strategic priorities and diplomatic outlooks.

As regional security dynamics become increasingly complex, particularly in critical sea lanes of the East China Sea, the Taiwan Strait, and the South China Sea, the extent to which Japan and South Korea can translate trilateral dialogue and joint exercises into effective bilateral and multilateral cooperation will be closely watched.