As China’s rise intensifies concerns about a potential invasion of Taiwan, North Korea’s nuclear arsenal – combined with its advancing missile capabilities – continues to pose a major regional challenge. Meanwhile, a second Trump administration in the United States could adopt a more isolationist stance, and the likelihood of closer cooperation between China and North Korea, or even a trilateral alignment with Russia, is becoming feasible.
In this evolving security landscape, the only established defense mechanisms for the U.S., Japan, and South Korea are the U.S.-Republic of Korea (ROK) Mutual Defense Treaty and the U.S.-Japan Security Treaty. While additional agreements exist, such as the U.S.-Japan Defense Guidelines and the U.S.-South Korea Missile Guidelines, these two treaties serve as the fundamental legal foundation for regional security. However, given that these frameworks were established over 70 years ago (with the U.S.-ROK Mutual Defense Treaty signed in 1953 and the U.S.-Japan Security Treaty originally signed in 1951 and revised in 1960), are they still sufficient to address the region’s evolving security threats?
One way to explore this question is through historical analogies. As Henry Kissinger noted, while historical events do not repeat themselves exactly, they can still provide valuable insights into patterns of international relations and decision-making. Certain historical precedents offer lessons on what should – and should not – be done in a given geopolitical context. To better understand the security vulnerabilities of contemporary East Asia, I have chosen the 1925 Locarno Treaties as a key example.
During the interwar period in Europe, tensions remained high as Germany, resentful of the Treaty of Versailles, sought to reclaim its status as a respected European power, while France remained wary of a potential German attack. In this climate, key European nations – including Germany, France, and Belgium – convened in 1925 to sign the Locarno Treaties, a set of seven agreements aimed at stabilizing European security. From this historical case, I have identified five key lessons that today’s policymakers in the United States, Japan, and South Korea should take into account.
Lesson One: The Danger of Partial Security Guarantees
One of the critical shortcomings of the Locarno Treaties was their failure to address Eastern Europe, while explicitly guaranteeing Western European borders. This omission inadvertently signaled to Germany that aggression in the east would not trigger Western intervention, ultimately emboldening Nazi expansionism.
The geopolitical landscape of East Asia in the 21st century has evolved significantly since the early 1950s, when the U.S.-ROK Mutual Defense Treaty and the U.S.-Japan Security Treaty were established. However, these treaties were primarily designed to protect South Korea and Japan, leaving a notable gap regarding a potential Taiwan contingency, as they do not explicitly include Taiwan within their security commitments. This omission could pose serious challenges in the event of a conflict over Taiwan.
Unlike in the early 1950s, when President Harry Truman ordered the U.S. Seventh Fleet into the Taiwan Strait to prevent both Mao Zedong from invading Taiwan and Chiang Kai-shek from launching an attack on mainland China, today’s China is far more powerful, and the possibility of an invasion of Taiwan has become a real and pressing concern for the U.S., Japan, and South Korea. Despite the existing security treaties, the lack of a coordinated framework for a Taiwan contingency remains a major strategic vulnerability.
Through interviews with South Korean defense officials and Japanese Self-Defense Force officials in 2022 and 2023, I found that in the event of a Taiwan contingency, North Korea could exploit the situation to its advantage. Furthermore, given South Korea and Japan’s differing security priorities – with Japan prioritizing the defense of the Taiwan Strait and South Korea focusing on countering North Korean aggression – both nations would likely allocate their military assets according to their respective strategic imperatives. This lack of coordination could lead to fragmented and inefficient responses.
To mitigate these risks, the U.S., Japan, and South Korea must develop a clear, structured plan detailing how they would coordinate their military and non-military assets in the event of a Taiwan contingency, whether Taiwan alone or a simultaneous crisis involving both the Taiwan Strait and the Korean Peninsula. While the Trilateral Security Cooperation Framework may already contain some provisions for such scenarios (the details remain classified), my discussions with defense officials from both Japan and South Korea suggest that more specific measures are needed to ensure a cohesive and effective response.
Lesson Two: The Illusion of Diplomacy Without Enforcement
The Locarno Treaties were initially hailed as a diplomatic success, yet they ultimately lacked a credible enforcement mechanism. When Germany violated the terms, as with its re-militarization of the Rhineland in 1936, Britain and France failed to respond, effectively undermining the treaty system.
Similarly, while the U.S.-ROK Mutual Defense Treaty and the U.S.-Japan Security Treaty explicitly reaffirm the United States’ commitment to defending South Korea and Japan from external military threats, new and unprecedented challenges have emerged. Among the most pressing is North Korea’s advancement in nuclear capabilities, particularly its miniaturized nuclear warheads mounted on long-range missiles. Whether intentional or not, this growing nuclear capability is likely to drive a wedge between the United States and its allies, potentially leading to nuclear decoupling between the U.S.-Japan and U.S.-South Korea alliances – thereby further eroding the credibility of U.S. extended deterrence.
If North Korea successfully develops a reliable MIRV (Multiple Independently Targetable Reentry Vehicle) technology – capable of unpredictable flight paths that can bypass U.S. missile defense systems and reach the U.S. mainland – Japan and South Korea may begin to doubt whether Washington would truly honor its security commitments in the event of North Korean military aggression. A similar dilemma confronted Western European countries between the 1960s and 1980s, though it was eventually resolved in their favor, as the Soviet Union refrained from fully exploiting this vulnerability and ultimately collapsed in 1991.
However, North Korea’s more unpredictable nature, coupled with its growing nuclear arsenal, could enable it to continuously test the limits (red lines) of Japan and South Korea through various provocations. This could lead to frustration and fatigue among policymakers in Tokyo and Seoul, potentially increasing the likelihood of greater concessions to North Korea.
To mitigate this risk, several potential solutions have been proposed, including nuclear sharing agreements, the reintroduction of tactical nuclear weapons to South Korea, or even South Korea developing its own nuclear arsenal. However, the core issue is not military capability, but rather the credibility of deterrence – specifically, whether the United States is willing to honor its commitment in a crisis.
Thus, it is crucial for the U.S., in close consultation with Japan and South Korea, to establish explicit red lines and to demonstrate – regularly and consistently – that North Korean violations will have concrete consequences. Additionally, Japan and South Korea must strengthen their conventional military capabilities to counter potential nuclear threats. In this context, bolstering Japan’s enemy base strike capability and reinforcing South Korea’s Kill Chain would be essential steps toward enhancing deterrence and maintaining regional stability.
Lesson Three: Balancing Alliances and Strategic Autonomy
The Locarno Treaties relied on Britain as a guarantor, yet Britain’s reluctance to use military force when necessary ultimately weakened the agreement. This underscores the inherent risks of relying too heavily on external powers for national security.
At this moment, it remains uncertain how the newly inaugurated U.S. President Donald Trump will approach the alliances with Japan and South Korea. Whether he adopts an isolationist stance or takes a more proactive role in defending both allies, it would be strategically prudent for Japan and South Korea to strengthen their own military capabilities to ensure they can independently deter or respond to external aggression – even if this process takes time.
As previously mentioned, in the event of a two-front conflict – with simultaneous crises in the Taiwan Strait and the Korean Peninsula – the United States’ military resources would be stretched thin. Under such circumstances, the military assets of Japan and South Korea would play a critical role in ensuring their own defense, making their independent military preparedness increasingly vital for regional stability.
Lesson Four: The Risks of Overlooking Nontraditional Threats
The Locarno Treaties prioritized territorial integrity but failed to address emerging economic, ideological, and hybrid threats, such as Nazi propaganda and economic manipulation. Similarly, today’s security environment demands a more comprehensive approach, encompassing economic security, cyber defense, and supply chain resilience.
A key lesson from recent history is that nontraditional threats can serve as a precursor to open conflict. For nearly a decade before its full-scale invasion of Ukraine in 2022, Russia engaged in hybrid warfare, employing cyberattacks, disinformation campaigns, political destabilization, economic coercion, and support for separatist movements. This underscores the need to take nontraditional threats seriously, as they can gradually erode stability and pave the way for direct military aggression.
The U.S.-ROK Mutual Defense Treaty and the U.S.-Japan Security Treaty primarily focus on defending South Korea and Japan’s territorial integrity. However, given the asymmetric threats posed by China and North Korea, security cooperation between the U.S., Japan, and South Korea must continue to expand beyond military deterrence. While recent trilateral efforts, particularly the 2023 Camp David Summit, have tackled issues such as technology security, cyber defense, and economic resilience, additional institutionalized mechanisms are required to enhance coordination against non-traditional threats. Failing to address these vulnerabilities could weaken the trilateral security framework, heightening strategic risks in the region.
Lesson Five: The Importance of Multilateralism and Trust
The failure of the Locarno Treaties was partly driven by growing distrust among European states, particularly as Germany and Italy shifted alliances, ultimately undermining collective security. Similarly, historical tensions between Japan and South Korea could pose challenges to trilateral security cooperation between the U.S., Japan, and South Korea.
As I highlighted in my doctoral dissertation, “Japan’s Security Contribution to South Korea, 1950 to 2023,” Japan-South Korea security cooperation has persisted for over 70 years (from 1950 to the present), despite periodic tensions stemming from historical disputes. Additionally, my research underscores that emerging threats – particularly North Korea’s advancing nuclear missile capabilities, which pose a direct risk to the U.S., Japan, and South Korea – have made trilateral security coordination even more essential.
Nevertheless, if domestic political tensions overshadow security priorities between Japan and South Korea for an extended period, it could have adverse consequences, even if the overall security cooperation remains intact. For example, both countries may focus on developing their respective defense capabilities independently, rather than in a coordinated manner. This could lead to resource inefficiencies, unnecessary redundancies, and strategic misalignment, ultimately weakening collective deterrence.
While proposals such as integrating Japan’s counterstrike capability with South Korea’s Three Axis System may seem too ambitious at this juncture, at a minimum, Japan and South Korea must prioritize security cooperation over historical disputes. Doing so would help prevent potential adversaries or regional spoilers from exploiting divisions between the two countries or miscalculating the situation, which could ultimately undermine regional stability.
Conclusion
It has been 100 years since the signing of the Locarno Treaties, yet their failure to prevent World War II continues to offer valuable lessons for today’s policymakers. If replacing the 70-year-old U.S.-ROK Mutual Defense Treaty and U.S.-Japan Security Treaty with an Asian equivalent of NATO remains unlikely in the foreseeable future, it would be prudent for policymakers in the United States, Japan, and South Korea to learn from the mistakes of the Locarno Treaties and consider the policy measures outlined above. By implementing these measures and applying the lessons of Locarno’s failure, the U.S., Japan, and South Korea would be better prepared to manage potential regional crises, reducing the risk of repeating the tragic consequences that the signatories of the Locarno Treaties ultimately faced.