Japan and South Korea do not have a military alliance akin to NATO. Nonetheless, a Japan-South Korea Acquisition and Cross-Servicing Agreement, or ACSA, would prove valuable in a period of crisis. It is most appropriate to understand ACSA as an agreement on logistics. It enables the two militaries to provide materials and services to one another in accordance with agreed conditions. Such support may include fuel, food, transportation, maintenance, medical support, communications support, repair services, and non-combatant assistance. This does not impose obligations for combat. Yet as the war in Ukraine demonstrated, modern warfare can be translated into a prolonged competition over stockpiles, transportation networks, repair capacity, drones, missiles, and air-defense systems.
ACSA will not be equivalent to creating a military alliance. The agreement would not automatically allow the Japanese Self-Defense Forces to enter the Korean Peninsula. Nor would it require South Korea to militarily support the United States and Japan during a Taiwan contingency. Nevertheless, ACSA matters—since logistics in modern warfare, as in previous eras, is far from a secondary matter. It is what enables military power to endure, especially during a protracted crisis.
Hence, Tokyo and Seoul should consider an ACSA that is carefully designed. An agreement could heighten the resilience of US-Japan-South Korea cooperation, strengthen deterrence against North Korea, and help in preparing for simultaneous contingencies in the Taiwan Strait and the Korean Peninsula. But it should be limited in scope, conditional, and transparent. If treated carelessly, it could trigger political backlash within South Korea while allowing China and North Korea to exploit it for propaganda purposes.
For Japan, ACSA would consolidate its role as a central security actor in the Indo-Pacific. So far, Tokyo has already signed ACSA with eleven partners including Australia and India. An agreement with Seoul would, in practical terms, create an important missing linkage and help both countries avoid a repeat of the 2013 South Sudan incident, which led to diplomatic tensions. Eventually, ACSA would institutionalize trilateral cooperation, reinforce deterrence against China and North Korea, and create a direct bilateral channel for logistics coordination during regional crises.
For South Korea, the issue is immediate. A full-scale Korean contingency would not be fought solely by South Korean troops and US Forces Korea (USFK). US reinforcements would be introduced from Japan, Guam, Hawaii, Alaska, and the continental United States. Similar to the 1950s Korean War, Japan-based facilities—such as Yokosuka, Sasebo, Kadena, Iwakuni, and Yokota—would play crucial roles in the fields of refueling, repair, transportation, command-and-control, medical evacuation, and maritime operations. Because South Korea would be the primary battlefield and has limited strategic depth, its defense would depend not only on its own territory but also on a broader rear-area logistics network across Japan and the Western Pacific.
A well-designed ACSA could help South Korea in several ways: it could accelerate the tempo of US reinforcement by reducing the administrative frictions between Tokyo and Seoul; operational sustainability would be strengthened through access to Japanese rear-area support especially for fuel, repair, and transportation; maritime resilience would also be enhanced, given that both countries are heavily dependent on sea lines of communication (SLOC) for energy, trade, and military logistics; and trilateral exercises would gain meaning from the legal and administrative foundation for crisis use.
The strongest case for ACSA emerges in a dual-contingency scenario. If North Korea heightens the pressure on the Korean Peninsula while China escalates tensions around Taiwan, American aircraft, naval forces, missile defense, intelligence, and logistics assets would be under severe strain. Under such circumstances, South Korea’s major task would not necessarily be to dispatch troops to Taiwan. More likely, Seoul would be required to maintain deterrence and defense on the Korean Peninsula while the United States manages a broader regional crisis. In that context, ACSA could serve as logistical insurance by ensuring access to Japanese rear-area support and lowering the overall dependence on Washington as the sole intermediary between its two Northeast Asian alliances.
Nonetheless, political risk is real. Due to the history of Japanese colonial rule in Korea, any form of security cooperation with Japan remains highly sensitive. South Korean critics could frame ACSA as a backdoor alliance, a step toward Japanese militarism, or as a mechanism for the Japanese Self-Defense Force’s introduction to South Korean soil. Meanwhile, North Korea would condemn it as evidence of an emerging US-Japan-South Korea military bloc. China may interpret it as part of a broader containment strategy.
For this reason, the agreement must incorporate clear safeguards.
First, ACSA should explicitly state that the Japanese Self-Defense Force is not authorized to enter South Korea’s territory, airspace, or territorial waters without Seoul’s prior request and explicit consent. Second, support should be conducted only when requested by one party and explicitly authorized by the other. Third, the agreement should clearly state that it should not request South Korea’s direct participation in a Taiwan contingency. Fourth, the initial scope should be centered on non-combat logistics, such as fuel, food, maintenance, medical assistance, communication, humanitarian support, evacuation of non-combatants, and training support. Ammunition, weapons systems, and other lethal devices should require separate approval. Fifth, the South Korean government should report the agreement to the National Assembly and publicly explain its limitations.
A phased approach would be politically most durable. The initial phase could include disaster relief, peacekeeping, counter-piracy, non-combatant evacuation, and logistics support for joint exercises. Subsequent phases could address Korean contingencies, maritime logistics, rear-area support, and US reinforcements.
If designed properly, ACSA need not constitute a political liability for Seoul. Rather, it could function as a practical instrument for deterrence and operational sustainability. Amid a deteriorating security environment in Northeast Asia, Tokyo and Seoul would benefit from a procedure that could function under duress.
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