Japan’s JS Unryu Submarine and U.S. Marines Conduct Joint Resupply Operations in Japan.

The Japan Maritime Self-Defense Force submarine JS Unryu (SS-502) joined U.S. Marines at Marine Corps Air Station Iwakuni on October 26, 2025, for a coordinated resupply operation. The exercise reflects growing logistical cooperation and undersea readiness between the two allies in the Indo-Pacific region.

According to information released by the U.S. Department of War, the Japan Maritime Self-Defense Force submarine JS Unryu (SS-502) arrived at Marine Corps Air Station Iwakuni, Japan, on October 26, 2025, to carry out a joint resupply operation with U.S. Marine logistics elements. The visit, though routine in appearance, marks a significant step in advancing interoperability between Japanese and American forces. Officials described the operation as part of a broader effort to strengthen allied undersea logistics and sustainment capabilities in the Western Pacific.
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The Japan Maritime Self-Defense Force submarine JS Unryū (SS-502), assigned to Submarine Division 5 of the Fleet Submarine Force based in Funakoshi, arrives at Marine Corps Air Station Iwakuni, Japan, to conduct a resupply operation, October 27, 2025. (Picture source: U.S. Department of War)

The Unryū’s visit to MCAS (Marine Corps Air Station) Iwakuni highlights expanded operational flexibility in the U.S.-Japan alliance. It demonstrates JMSDF’s ability to sustain submarine operations beyond traditional homeports and reflects the U.S. commitment to integrating undersea platforms into shared logistics networks. This advancement directly supports the evolution of distributed operations in the Western Pacific.

To better understand the platform involved, it is notable that JS Unryū, commissioned in March 2010, is the second unit of the Sōryū-class diesel-electric submarines, built by Kawasaki Heavy Industries. With a submerged displacement of around 4,200 tons and a length of 84 meters, the Sōryū-class ranks among the world’s most advanced non-nuclear submarines. Early units in the class, including Unryū, are equipped with air-independent propulsion (AIP) systems based on Stirling engines. This system enables significantly extended submerged endurance compared to traditional diesel-electric boats, allowing for stealthier patrol patterns and longer mission durations, particularly in contested waters such as the East and South China Seas.

The Unryū submarine is equipped with six 533 mm bow torpedo tubes capable of firing both Type 89 torpedoes and UGM-84 Harpoon anti-ship missiles. The class features a highly automated combat system, a low-acoustic-signature hull design, and an X-shaped rudder to enable precise maneuvering in shallow or congested maritime zones. While later Sōryū-class boats transitioned to lithium-ion battery technology, AIP-equipped hulls like Unryū continue to provide critical operational relevance for long-duration missions under strict emission control (EMCON) conditions.

The October 27, 2025, port visit at Iwakuni enabled JMSDF crews to execute a range of shore-based support procedures. These included auxiliary resupply, environmental systems maintenance, and communications integration. Although specific replenishment details were not publicly disclosed, such drills typically encompass tasks like battery maintenance, fresh water supply, waste management, and hull cooling system checks. U.S. Marine Corps personnel and U.S. Navy support elements reportedly assisted in coordinating the operation. This marked a rare instance of joint logistics support for a submarine at a Marine Corps aviation facility.

Strategically, the operation reflects a subtle but significant shift in alliance posture. In recent years, both Japan and the United Sttaes have emphasized the importance of enhancing bilateral logistics, mobility, and access. This priority is reflected in frameworks such as the U.S.-Japan Alliance Coordination Mechanism and Japan’s 2022 National Defense Strategy. The ability to resupply and support JMSDF submarines at U.S. forward bases introduces a deeper level of deterrence in the region. It also supports the U.S. Marine Corps' Expeditionary Advanced Base Operations (EABO) concept, which focuses on dispersed, flexible basing nodes, including air stations, to support multi-domain forces like submarines operating in contested maritime zones.

In the context of China’s growing naval presence in the East China Sea and the broader Western Pacific, exercises like this carry both operational and symbolic significance. They send a clear message to regional observers: allied submarine forces are not only capable of operating together but are also able to sustain each other through integrated infrastructure. This capability could increase deployment tempo and reduce turnaround times in high-stakes situations, enhancing readiness and deterrence posture.

JS Unryū’s visit demonstrates a high level of logistical adaptability and may serve as a blueprint for future undersea sustainment operations. These may include Japan’s newest Taigei-class submarines as well as U.S. attack submarines operating under INDOPACOM. The operation also aligns with the trilateral logistics agreement signed in 2025 by Japan, the United States, and Australia, which set protocols for shared resupply, maintenance, and infrastructure access for naval forces, including undersea platforms.

While military planners and defense analysts might initially regard the event as routine, its broader implications reveal a quiet transformation in allied maritime strategy. It highlights a future where the undersea domain is defined not just by stealth and strike capabilities, but by logistics, endurance, and sustained presence across critical maritime chokepoints. In such a battlespace, a submarine's silent arrival at a joint-use harbor like Iwakuni may signal more than a routine port call. It may mark a decisive shift in how the U.S. and Japan prepare for a new era of Indo-Pacific deterrence.