In East Asia, the strategic environment is no longer defined by an isolated crisis. It is increasingly shaped by simultaneity. A dual contingency–China attempting to occupy Taiwan while North Korea escalates tensions on the Korean Peninsula–is no longer a remote possibility. What makes the scenario more dangerous is not the mere existence of these two theaters, but the interaction between them; they mutually reinforce each other, compress decision-making timelines, and diffuse allied resources.
For decades, the United States functioned as the central pillar of East Asian security. The alliance network with Japan and South Korea deterred large-scale military conflict and buttressed regional stability for more than seventy years. However, the structure was based upon an implicit assumption; that crises can be managed sequentially while the United States would maintain sufficient strategic reserves to dominate any single theater. Yet, this assumption is eroding.
The newly evolving global environment is illustrating a different reality. A Taiwan contingency would require an immediate and sustained U.S. military engagement–particularly the forwardly deployed naval and air assets in the Indo-Pacific. Meanwhile, North Korea might exploit the crisis and test South Korea’s defense preparedness through missile launches, limited infiltration, or broader conventional types of escalation. Beyond East Asia, the United States is faced with persistent commitments in Europe, while the probability of a Russian invasion against a NATO member state also cannot be ruled out. The result is a structured problem. The United States may no longer offer decisive reinforcements across all theaters.
In this scenario, the burden of safeguarding regional security would inevitably be transferred to frontline allies. For Japan and South Korea, this is not simply a problem of alliance burden sharing. Rather, it is a matter of strategic survival. If U.S. reinforcements are delayed, diverted, or diluted, the ability of these two countries to coordinate their defense efforts will determine whether deterrence is maintained or not in the early stages of the crisis.
Nevertheless, Japan-South Korea security cooperation remains constrained by historical grievances and political sensitivities. Disagreement on wartime history, territorial disputes, and domestic political narratives has repeatedly hampered attempts for deeper bilateral cooperation. These issues are real and cannot be neglected. However, it should be weighed against the imminent reality: the structural strain originating from the regional security environment barely allows room for long-term bilateral friction.
The logic is straightforward. In a dual contingency scenario, both Japan and South Korea do not have the luxury to act independently. The geographical and operational reality of East Asia makes both countries’ security interdependent. United States Forces Japan (USFJ) is a necessary component of U.S. reinforcement to the Korean Peninsula. Conversely, South Korea’s stability is essential for maintaining a broader regional balance that underpins Japan’s defense. Fragmentation between the two countries would not only undermine deterrence but also invite opportunism from adversaries that are increasingly capable of exploiting gaps in coordination.
Then what should be done?
First, Japan and South Korea should institutionalize real-time intelligence sharing that goes beyond the limits of the existing agreement. To be sure, general security of military information agreement (GSOMIA) offers one form of a framework. Nevertheless, it is insufficiently used while being politically fragile. Both countries should expand it into a robust and automated intelligence-sharing mechanism that is centered on missile tracking, drone activities, and maritime domain awareness (MDA). This requires not only technological integration, but also political will to protect the mechanism from diplomatic volatility.
Second, both countries should establish a standing bilateral–or a trilateral one that includes the United States–planning organization that is dedicated to a dual contingency scenario. The current coordination mechanism is largely ad hoc and reactive. A permanent planning structure would enable the development of joint crisis planning, wargaming, and shared operational concepts. This is especially important for escalation management where misalignment could lead to delay or contradicting responses.
Third, interoperability should be moved from rhetoric to reality. This means the alignment of C2 systems, communication protocols, and logistics support systems. Joint exercises should develop into practical ones that simulate high-intensity multi-theater conflict, instead of symbolic demonstrations. For example, an air and missile defense exercise that integrates Japan’s Aegis destroyers and South Korean ground-based systems, or aligning Japan’s enemy base strike capability to South Korea’s Three-Axis System could structure a multi-layered and resilient defense architecture.
Fourth, Japan and South Korea should deepen their cooperation in the field of defense production and maintenance. Modern warfare is not about mere platforms, but a matter of capabilities that could sustain operations in long stretches of time. Ammunition stockpiles, spare parts, and maintenance capabilities are all critical bottlenecks. In particular, by coordinating production and supply chains in areas such as precision guided munitions and missile defense, both countries could strengthen collective resilience. This would also reduce the overall dependence on U.S. resupply in the early stages of the war. After all, Japan’s industrial capacity helped the US warfighting efforts during the Korean War in the early 1950s. In the contemporary era, Japan and South Korea could create meaningful synergy that could meet the potential surge in demand originating from two theaters.
Fifth, maritime cooperation should be deepened. The sea lines of communication (SLOC) that link Japan and South Korea would be crucial not only for economic reasons, but also from the standpoint of military logistics. In crisis, these routes would likely be contested. Joint maritime patrols, sharing of surveillance systems, and cooperation on anti-submarine warfare (ASW) would contribute to safeguarding these lifelines. This would be particularly invaluable considering the aggrandizement of Chinese and North Korean naval forces.
Sixth, both countries should establish a joint countermeasure mechanism against grey-area activities. North Korea’s use of drones, cyber operations, limited provocations, and China’s coercive maritime activities often exist below the threshold of conventional conflict. Without coordination, responses against these activities could appear inconsistent and inefficient. A joint framework for responding to grey-area zone challenges would reinforce deterrence by signaling cohesiveness and resolve.
Finally, political leadership in both countries should provide a clear narrative that frames cooperation as a strategic necessity, instead of a concession. Particularly in South Korea, historical issues with Japan remain an unignorable constraining element. Leaders of both sides should ceaselessly persuade their respective publics that stronger security cooperation is about addressing the present and preparing for the future, instead of overlooking the past. Absent these narratives, even the most well-designed policies cannot gain the necessary traction.
These measures do not necessarily require a formal alliance between Japan and South Korea. Moreover, they do not demand the resolution of all historical disputes. What is most needed is fully grasping that the strategic environment has fundamentally altered. Since the challenges of dual contingency are paramount, while the margin of error is very narrow, it would be grievously risky to let political frictions dictate security policy.
The United States will remain an indispensable ally, at least in the foreseeable future. However, in an era when global burdens are accumulating, the ability to play the role of a sole guarantor for regional security is uncertain. Therefore, Japan and South Korea should assume greater responsibility–not individually, but collectively.
The alternative is a fragmented defense posture in an area where more coordinated, opportunistic, and capable adversaries exist. In such an environment, the price of inaction would be assessed not as diplomatic, but as a strategic failure.
[Image credit: OpenAI/DALL·E-generated illustration.]
The views and opinions expressed in this article are those of the author.
Dr. Ju Hyung Kim currently serves as the president at the Security Management Institute, a defense think tank affiliated with the South Korean National Assembly. He has been involved in numerous defense projects and has provided consultation to several key organizations, including the Republic of Korea Joint Chiefs of Staff, the Defense Acquisition Program Administration, the Ministry of National Defense, the Korea Institute for Defense Analysis, the Agency for Defense Development, and the Korea Research Institute for Defense Technology Planning and Advancement. He holds a doctoral degree in international relations from the National Graduate Institute for Policy Studies (GRIPS) in Japan, a master’s degree in conflict management from the Johns Hopkins University School of Advanced International Studies (SAIS), and a degree in public policy from Seoul National University’s Graduate School of Public Administration (GSPA).
