The Quiet Thaw: Japan and South Korea Rebuild a Fractured Alliance

After years of frozen naval ties, Tokyo and Seoul are quietly stitching their defense relationship back together — driven by shared anxieties over North Korea, China, and an uncertain American security umbrella.

Nine years. That's how long it had been since Japanese and South Korean warships trained together — a freeze triggered by the 2018 radar incident, when a South Korean destroyer allegedly locked its fire-control radar onto a Japanese patrol aircraft, triggering one of the worst diplomatic crises in postwar relations between the two U.S. allies.

On June 7, 2026, that freeze finally cracked.

The Japan Maritime Self-Defense Force's Kongo-class Aegis destroyer JS Kongo and the Republic of Korea Navy's tank landing ship ROKS Cheon Ja Bong conducted a bilateral Search and Rescue Exercise (SAREX) west of Japan's Goto Islands. It was the first SAREX since 2017 — a modest drill on paper, but freighted with strategic significance. The exercise included not just search-and-rescue operations but LINKEX (tactical data link training), cross-deck helicopter operations, and a photo exercise — activities that build the muscle memory of interoperability.

Japanese Defense Minister Shinjiro Koizumi called it "the beginning of a new chapter."

The thaw has been building. At the 2024 Shangri-La Dialogue, defense ministers agreed on measures to prevent a repeat of the 2018 incident. By January 2026, a Yokosuka ministerial meeting formally confirmed SAREX resumption. In May, Prime Minister Sanae Takaichi and President Lee Jae Myung held a summit in Andong, South Korea, discussing energy security — including swap arrangements for crude oil and LNG — and the delicate balancing act both nations face between their U.S. alliance obligations and the need to avoid antagonizing China.

Yesterday, the pace accelerated. On June 28, Koizumi met his South Korean counterpart Ahn Gyu-back in Seoul for the sixth round of bilateral defense talks. They reaffirmed commitment to the denuclearization of the Korean Peninsula and pledged closer coordination on regional stability — both bilaterally and through Washington. South Korea's defense ministry noted the "grave security environment" in a pointed statement.

The backdrop is unmistakable. North Korea's advancing nuclear and missile programs. China's expanding naval presence. Uncertainty over future U.S. alliance commitments under a transactional Trump administration. And the Strait of Hormuz — where Washington wants allied naval contributions, and both Tokyo and Seoul are weighing public opinion against alliance pressure.

Challenges remain. The Acquisition and Cross-Servicing Agreement (ACSA) — which would allow mutual logistical support between the two militaries — remains unresolved. Historical sensitivities, particularly in South Korea, continue to constrain deeper cooperation. And the two governments diverge on China: Seoul wants to keep the U.S.-South Korea alliance disentangled from any Taiwan contingency, while Tokyo has drawn a harder line.

But the direction of travel is unmistakable. Two neighbors, one alliance, and a rapidly shifting strategic landscape are pushing Seoul and Tokyo closer together — not with speeches, but with ships at sea.

Sources: Naval News, The Business Standard / Reuters, The Diplomat