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Optimizing the US-Japan and US-Korea Alliances in the Maritime Domain

Updated: Jul 2, 2022

By Eli Cook



The Indo-Pacific maritime environment is a critical arena of international security concerns. As the primary domain through which local state interaction occurs, control of the sea, or lack thereof, defines regional order. Many Indo-Pacific nations rely either heavily, or near wholly, on seaborne trade to sustain their economies. Japan, Australia and other island nations use vital sea routes, or SLOC (Sea Lines of Communication) as lifelines to connect them with the world at large. Similarly, states that are connected to the Asian mainland are heavily reliant on SLOC as well, either due to the intrinsically high efficiency of seaborne trade, or in the case of the ROK (Republic of Korea), because the bordering DPRK (Democratic People’s Republic of Korea) blocks any land based trade routes. In this sense South Korea is effectively a geopolitical Island nation, similar to Japan.

Beyond economic considerations, the maritime domain plays other geopolitical roles. State interaction within the Indo-Pacific, namely US-China hegemonic competition, and a variety of other regional disputes, is inherently constrained by the same factors that generally restrict regional trade. Lacking extensive land borders and easily navigable terrain, conflict between maritime nations is pushed offshore, and by extension into the same SLOC upon which the local economies relies. The maritime nature of the Indo-Pacific confines different aspects of state interaction into a series of relatively narrow waterways, conflating issues and raising stakes.


Seeking to protect the trade that powers their economies, both South Korea and Japan have adopted naval strategies and force structures aimed at preserving sovereignty and control. Japan has long possessed a world class navy with a variety of large warships, while the ROK has undertaken a massive shipbuilding program following the end of the Cold War. National planning documents suggest this trend is likely to continue.

South Korean and Japanese maritime approaches exist alongside US national strategic interests within this theater. The same trade that is vital to Indo-Pacific nations is also of critical importance to the US economy. Further, great power competition between the United States and China remains a priority concern for all regional actors. As a revisionist power, China seeks to corrupt and corrode the international norms and institutions that protect the free flow of seaborne trade, to an extent replacing them with counterparts of Chinese origin.

While both Japan and the ROK have critically improved their capabilities within the maritime domain, they have been unable to match the speed and size of Chinese naval modernization and expansion efforts. With each year, the balance of power within the Indo-Pacific shifts in China’s favor, as the state’s economy and technological base develop. This presents a long term strategic problem for the United States and its allies.

Given these conditions, there is ample room for Japanese and South Korean participation in US led strategic security frameworks for controlling the Indo-Pacific maritime domain. All three states share a common interest in preserving the rules-based international order to ensure the economic activity that has uplifted post-war Asian economies. The United States also directly benefits from increased ROK and Japanese naval activity as a counterweight to growing Chinese influence.

However, while both are successfully engaging with the United States to differing degrees, there remain significant barriers preventing their optimal cooperation for furthering American policy. In the maritime domain this has broadly taken two forms. The first is differences in approach to freedom of navigation issues. Japan has taken a strong public position supporting US FONOPS (Freedom of Navigation Operations) missions within the region, arguing they are crucial for maintaining the rule of maritime law. By establishing precedent for the passage of military assets through disputed claims, Japanese policymakers feel they are better positioned to utilize these assets to protect their own interests.

Contrarily, South Korea has taken a far more subdued stance on the issue, generally publicly supporting these endeavors but refraining from the same level of commitment as Japan. Between security distractions from the DPRK and more cohesive trade integration with China, the ROK has less room for political maneuver. Both states also have some limited national policies of restricting access to territorial waters, counter to the underlying principles of freedom of navigation

The second area of divergence of Japanese and ROK policy from American strategic designs involves the recognition and control of various disputed maritime claims. While the two states likely support portions of the American approach to Chinese rhetoric, they remain wary of resolving these challenges. Both Japan and South Korea possess disputed holdings of their own within the surrounding seas. Establishing precedents through taking a strong position may lend greater weight to opponents of their own claims. Of note, Japan and the ROK assert competing claims over the Tsushima strait regarding sovereignty of the Dokdo/Takeshima maritime features/islands. This dispute, in particular, is highly damaging to American goals.

Policy Recommendation

To further the strategic outcomes favored by the United States within the Indo-Pacific, the US should continue pre-existing strategies emphasizing freedom of navigation and supporting a rules-based international maritime order. Aspects of Japanese and South Korean policy in this regard are at odds with the United States. Neither has fully adopted a policy approach embracing international maritime conventions. Optimizing the US-Japan and US-ROK alliances for the benefit of American Indo-Pacific strategy centers on fixing this disconnect.

There are multiple avenues to pursue change. For one, the United States should increase diplomatic pressure on Japan and South Korea to resolve their disputes in a manner consistent with international law. There are serious impediments to this goal, in part originating from historical enmity between the two. That said, maintaining a diplomatic dispute with a potential partner over historical and public opinion related concerns is unconducive to effective strategy. Both states would free up resources and further ensure their own fundamental goals by attempting to de-escalate tensions. Resolving differences under the guise of international law may make potentially unfavorable outcomes more palatable to their respective general publics. Regardless, this will likely prove a tenuous proposition.

Should outstanding maritime claim issues on the part of South Korea and Japan be resolved, the two are far better positioned to support US FONOPS within the region. These operations speak to both states' paramount strategic goal of ensuring the free flow of trade, supported by the deployment of military assets when necessary.

Resolving these contradictory positions would allow for Japan and the ROK to play a much more active role in countering broader Chinese revisionism. As developed nations with large technology-driven economies, both South Korea and Japan are capable of sustaining substantial military forces. This is untrue for many Southeast Asian nations, who also find themselves at odds with China. Countering the so-called Chinese 9-dash line claims over wide swaths of the South China Sea is important to ensuring that the waterway remains a public good, available for use by all Indo-Pacific nations, as well as to the benefit of the United States. Should Japan and the ROK resolve their own claims through legally sanctioned methods, their actions would juxtapose favorably against frivolous Chinese approaches in terms of perceived legitimacy.

To accomplish these goals, the United States must seek new methods for making clear to South Korean and Japanese policymakers that standardizing their approaches would be beneficial to all involved parties. While the ROK may never be particularly well positioned to respond strongly to Chinese threats regarding maritime sovereignty, taking action to clarify their strategic position would send a strong message and potentially create more space for diplomatic maneuver. Japan already enjoys freedom to speak more directly against China, but would have its position strengthened by the introduction of reinforcing rhetoric from the ROK. All developments of this nature lend themselves to a US Indo-Pacific strategy centered on engagement and norms protection.


The Indo-Pacific is a region characterized by maritime conflict and uncertainty, challenging the vital flow of seaborne trade beneficial to all regional economies. The economic lifelines connecting South Korea and Japan to the rest of the world exist under threat of disruption from destabilizing Chinese action. While China itself benefits greatly from the continued flow of seaborne trade, a future where that trade flows as dictated by China is an existential threat to the wellbeing and independence of these two US allies, and to a degree the United States itself. However the US-Japan and US-ROK alliances are not at present optimally configured for combatting revisionist influences in this sector, as both Asian states themselves maintain some degree of illiberal maritime claims.

In order to further ensure the free flow of seaborne trade through the Indo-Pacific and to the United States, the US must adopt new strategies to convince South Korea and Japan that resolving these illiberal claims is in their own strategic best interest. Should these US allies more effectively integrate with American strategy, the region as a whole will benefit, even if claims are not resolved in one particular side’s favor. This task is not simple, especially in light of a variety of political concerns, but is key to ensuring a future and open Indo-Pacific for benefit to all.


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