The United States and Japan Still Benefit From Complementary Maritime Capabilities

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Editors note: This essay is the seventh in a series of eight articles, “Maritime Strategy on the Rocks,” that examines different aspects and implications of the recently released tri-service maritime strategy, Advantage at Sea: Prevailing with Integrated All-Domain Naval Power. Be sure to read the firstsecond, thirdfourth, fifth, and sixth articles. We thank Prof. Jon Caverley of the U.S. Naval War College for his assistance in coordinating this series.


Japan may be moving to a more independent maritime force structure — will that make the U.S.-Japanese alliance stronger? Responding to growing regional security challenges, Tokyo has announced plans for new maritime capabilities, including shipboard fixed-wing aviation, amphibious forces, unmanned underwater vehicles, and others. While probably intended to give the Japanese government more options, the move toward capabilities more similar to those of the United States could mean a shift away from a complementary force structure between the allies. The historically complementary maritime structure makes the most of dollars, yen, personnel, and expertise. Such efficiency has maximized the alliance’s “capabilities for effective warfighting,” which Jeffrey Hornung argues is its foundation for deterrence. The United States and Japan need to appreciate what Japan has contributed to the maritime alliance, and proceed carefully to preserve warfighting capability.

Complementary Maritime Capabilities

The complementary security relationship between Japan and the United States is often likened to that between a shield and a sword. This analogy holds in the maritime domain, where Japan has developed significant reconnaissance, mine, and submarine forces (the shield) while the United States has maintained carrier strike, expeditionary strike, and special warfare forces (the sword). Alessio Patalano pointed to the benefits of such an arrangement, noting Adm. Thomas B. Hayward’s observation that the U.S. Navy and Japan Maritime Self-Defense Force (JMSDF) coordinated in the 1970s “so that they do not duplicate each other’s efforts.” On the other hand, the JMSDF force structure may too “skewed” for it to be considered an “independent navy,” as People’s Liberation Army scholar Dan Hua contended in work cited by Prof. Toshi Yoshihara.



The allies still maintain complementary maritime forces around Japan. For example, in the anti-submarine warfare, anti-ship warfare, and maritime reconnaissance missions, the JMSDF fields 74 P-3 and P-1 maritime patrol and reconnaissance aircraft, 81 patrol helicopters, 20 submarines, and 48 destroyers and frigates capable of anti-submarine and anti-ship warfare. The U.S. Navy typically deploys around Japan 16 to 20 fixed-wing patrol aircraft, two squadrons of radar-equipped patrol helicopters, and approximately 10 to 14 cruisers and destroyers. There is also a large difference in the number of counter-mine assets, with the JMSDF operating 10 minesweeping helicopters and 24 minesweeping ships, compared to the U.S. Navy’s detachment of minesweeping helicopters and four minesweeping ships. Thus in rapidly bringing anti-submarine or counter-mine assets to a conflict near Japan, the JMSDF possesses a comparative advantage over the U.S. Navy. The JMSDF also operates five replenishment ships and an extensive search and rescue service. To these we should add the capabilities of the Japan Coast Guard and other government agencies, which increase maritime reconnaissance and add a law-enforcement capability.

The U.S. Seventh Fleet maintains capabilities around Japan that Japan either does not field or possesses to a much lesser extent. In addition to obvious differences such as an aircraft carrier and strike fighter-equipped carrier air wing, the U.S. Navy deploys EA-18Gs capable of escort jamming, and most surface ships can strike land targets on land. Together with the Marine Corps Third Marine Expeditionary Force, the United States maintains in Japan a full-fledged expeditionary strike capability.

Is Japan Investing in Duplicate Forces?

Faced with an evolving security environment, including “changes in the power balance due to further expansion of national power by China and other countries,” Japan is investing in some maritime capabilities similar to those of the United States. The rationale appears in the current National Defense Program Guidelines, which state, “Defense capability is the ultimate guarantor of Japan’s national security … Japan must strengthen this capability on its own accord and initiative.” An October 2020 roundtable on the future of Japanese security and defense in the Texas National Security Review considers how Japan might adjust its defense capability. Two major programs are an amphibious capability and fixed-wing aviation on Izumo-class destroyers, discussed by Benjamin Schreer and Ryo Hinata-Yamaguchi respectively.

Both overlap with U.S. capabilities. The amphibious program centers on the Amphibious Rapid Deployment Brigade, supported by both JMSDF and Japan Ground Self-Defense Force (JGSDF) assets. Once fully buttressed by doctrine and training, this will give Japan an amphibious capability similar to some maintained by the Seventh Fleet-Third Marine Expeditionary Force team. The shipboard fixed-wing aviation program will upgrade two destroyers to multi-function platforms capable of temporarily embarking F-35B fighters. This would give Japan a capability similar to USS America, deployed to Sasebo and embarking Marine Corps F-35Bs.

The Japanese Ministry of Defense has also announced research in unmanned underwater vehicles and maritime unmanned aerial systems. One could imagine that other capabilities may be considered, such as tying the prioritized fields of cyber, intelligence, and electromagnetic warfare into an information warfare community; new deployment possibilities for maritime special operations forces; maritime electronic attack; or strike capabilities from ships, submarines, or maritime aircraft. These, too, are likely capabilities that duplicate those of United States forces around Japan.

The sea-based ballistic missile defense mission is a special case. Japan and the United States deployed capabilities to the region nearly simultaneously, with Japan demonstrating an intercept from JS Kongo in 2007, three years after the U.S. Navy activated USS Curtis Wilbur as the first ballistic missile defense tracking ship. The two efforts are roughly regionally equivalent in size and capability, and thus duplicate each other. Duplication may be necessary, as both capitals need control over a strategic defensive capability whose employment has to be ordered very quickly. The forces are also at least potentially complementary. They maintain a high degree of interoperability with each other and use multi-mission ships which could be deployed for other purposes. In particular, a persistent Japanese ballistic missile defense capability could free up multi-mission JMSDF destroyers, and potentially also U.S. Navy assets, for other missions.

Is Japan Divesting of Complementary Forces?

Japanese investments in new maritime capabilities would divert money and personnel from elsewhere. While Japan’s defense budget has grown over the past nine years, the growth has not been of a scale that could fund new capabilities without trade-offs. Furthermore, the JMSDF share of the budget has remained static or declined, as has the JMSDF’s percentage of an unchanging number of Self-Defense Force personnel. The JMSDF is not likely to gain significantly increased budgets or personnel for new capabilities, thus a new capability will probably mean internal cuts elsewhere.

If the JMSDF cuts legacy maritime capabilities to make way for new ones, it could reduce capabilities that have until now complemented those of the United States. For example, the JMSDF could be forced to accelerate retirements of P-3 patrol aircraft without fully replacing them with newer P-1 aircraft. While Japan is commissioning new fiber-reinforced plastic-hull minesweepers, it could downsize the overall minesweeper fleet. A recently-launched multi-mission destroyer (“FFM”) and planned   unmanned vehicles are intended to take up some of the counter-mine workload, but the multi-mission nature of the destroyer, its reduced manning, and the new technology behind planned counter-mine capabilities could introduce risk to the mission.

There has been little public discussion of which capabilities might be jettisoned to fund new ones. Some administrative restructuring has been announced — for instance the JMSDF Print Supply Unit was merged into the Tokyo Service Activity — but such reforms seem insufficient to pay for desired new capabilities. There is a risk that Japan would not formally cut programs but instead allow capabilities to languish with inadequate funding or exhausted personnel. Though such a policy has not been announced, one could imagine a temptation to fund new capabilities through cuts to less visible programs, such as ordnance reserves or spare parts. As the Japanese fleet has been expanding, the Ministry of Defense has announced few plans to build piers, replenishment ships, or other logistics infrastructure to support the fleet. The cost to the alliance for new maritime capability may be hidden. 

Does the United States Need to Be Concerned? 

The United States has an interest in both Japanese maritime capability itself, and in how it fits with American capability. In 2019, the U.S. Naval War College hosted a conference in Tokyo to encourage thinkers to “do a better job of incorporating other states’ interests and capabilities” into U.S. Navy strategic planning. The 2020 Advantage at Sea strategy released by the U.S. Navy, Marine Corps, and Coast Guard calls for allies and partners to “be ready and willing to bring capability and capacity to operations across the competition continuum.” Japan has interests, capability, and capacity across the competition continuum, but how will changes to those affect the alliance maritime capability as a whole?

The United States should first understand the assumptions behind Japanese maritime investments and divestments. Is Japan motivated primarily by a political desire for a more independent maritime strategy, or are there also military calculations driving the changes?

There are political motivations for a more independent Japanese maritime power. It can be difficult to justify a “skewed” force structure to the people paying for it, and building a more well-balanced, stand-alone maritime force — within constitutional constraints — is itself a goal for some politicians, both as an instrument of independent policy and as a matter of pride. Visible and easily understood capabilities may be more attractive to the public than asymmetric or support capabilities, and new programs can more clearly demonstrate the government’s resolve to defend Japan. A move toward independent capability can also be a signal that Japan questions America’s commitment, at least in certain circumstances — although unlike South Korea, Japan has announced nothing like an independent deterrent capability such as catastrophic retaliation.

Aside from providing Tokyo with more options, each of the announced or conceivable enhanced maritime capabilities has some military merit in the continuum of competition. Japan seeks to shape its security environment and may value peacetime presence over less-visible capabilities. Or Japan may estimate that the most dangerous phase is a “gray zone”, for which its complementary capabilities are less well-suited. It is worth noting that the National Defense Program Guidelines, as with other strategy documents, stop short of discussing conflict with the People’s Republic of China, but rather focus Self-Defense Force efforts on the crisis phase: “Japan will calmly and firmly deal with Chinese activities at sea and in the air around Japan.” Should there be a conflict involving Japan, Japanese planners may worry that United States forces will fight on a timeline or from locations different than those needed by Japan, necessitating a small Japanese capability that seemingly overlaps with that of the U.S. Navy and Marine Corps. Or in pursuing new capabilities at the expense of legacy ones, perhaps Japan perceives unaddressed changes in relative threat across missions.

A more independent Japanese security policy creates strategic and political questions, but Washington also needs to consider the effects on maritime capability. For instance, what if Japan were to grow a limited shipboard fixed-wing aircraft capability at the expense of counter-mine capability? If Japan were to fight alone, the addition of powerful shipboard fighters would help to round out Japanese options. Perhaps Japan would consider the decreased counter-mine capability an acceptable price to pay. However, if the allies fight a maritime campaign together, the picture would look different.

The U.S. Navy’s push for “interchangeability” built around the United Kingdom’s Queen Elizabeth-class carrier has merit for two navies with global missions, a wide variety of shared adversaries, and a class of ship designed from scratch with integration in mind. However, the U.S.-Japanese bilateral maritime picture differs from that of the U.S.-U.K. equivalent in important ways. With the exception of critical support missions such as logistics and mine countermeasures, Japan does not expect to fight alongside the U.S. Navy or Marine Corps in a maritime conflict far from home, or against less capable adversaries in the face of whom maritime superiority could be assumed. But Japan may need to fight alongside the United States closer to Japan, where in maritime conflict the United States assumes the People’s Liberation Army (Navy) will set the bar as a high-end maritime threat. F-35B fighters are designed to operate in high-threat areas, and the addition of two Japanese multi-mission destroyers with small, temporary fighter detachments could have a modest impact on the bilateral maritime force’s ability to conduct survivable air defense, strike, anti-ship, and other maritime missions. On the other hand, the destroyers themselves would become high-value units needing to be defended, and the operation of fighter detachments could diminish the destroyers’ ability to conduct anti-submarine warfare. Were Japan to pay for the shipboard fighter capability by reducing its minesweeper fleet, a reduction of even five or six minesweepers could have a major effect on the ability of the bilateral maritime forces to sustain maritime and logistics operations into and around Japan.

Other potential Japanese investments might impact bilateral maritime force structure differently. Despite strained manning, a JMSDF Information Warfare Community would benefit both countries, as the scale of maritime cyber, electromagnetic, and other threats in the information realm demands bilateral action. And while the United States has fielded numerous unmanned aerial systems and is advancing research and development in surface and subsurface systems, Japan possesses unique local knowledge and advanced technologies that could add quality and quantity to bilateral surveillance, anti-submarine warfare, and mine warfare missions.

Alternatives For Japan

New capabilities that broadly overlap with those of the United States could be tailored to increase their usefulness both for Japan and within the alliance. In his analysis of Japanese amphibious capability, Schreer asks why Japan would develop an Amphibious Rapid Deployment Brigade intended to mimic legacy capabilities of the U.S. Marine Corps, recommending that amphibious forces instead be redesigned as “elite amphibious commando units”. Rapidly deployable and locally expert, such units would create a comparative advantage within the alliance.

Japanese development of some capabilities similar to those of the United States could prove especially valuable if designed to be geographically complementary. For example, both allies possess considerable surface and subsurface surveillance capabilities, but as the demand for these continues to grow, improvements in Japanese capability could benefit the alliance if Japan can provide increased coverage in certain areas, allowing the United States to concentrate assets elsewhere.

Japanese maritime development need not be a zero-sum game. The cost of new maritime capabilities could potentially be borne by the Japan Air Self-Defense Force, JGSDF, coast guard, or others. For example, the Japan Coast Guard recently demonstrated a contractor-operated maritime unmanned aerial system out of the JMSDF Hachinohe Air Station. If the coast guard purchases a maritime surveillance system, and command and control can be arranged such that Japan Coast Guard capability also benefits the JMSDF, maritime capability could be grown without a compensatory divestiture. 


Both Japan and the United States rely on the combined power of their deterrence, and both countries benefit from the extra capability produced when either generates forces based on comparative advantage. Responding to political imperatives, Japan is likely to make changes to its maritime force structure that will alter the historically complementary relationship with the United States. To the extent that Washington wants Japan to have a unilateral edge in a regional conflict, the United States should greet its ally’s developments with enthusiasm. But the United States needs to appreciate how it still benefits from the complementary nature of Japanese maritime capability, and as that capability changes, Washington needs make adjustments in its own planning. Japan also needs to understand the impacts of its changes. Where Tokyo invests in duplicate capabilities, bilateral consultation is needed to integrate the capabilities into effective strategies. Where Japan divests, the United States needs to recognize and respond. In the competitive Pacific, neither Japan nor the United States can afford parallel campaigns.



Capt. Jim Hartman is the U.S. Navy Liaison Officer to the Japan Maritime Self-Defense Force in Tokyo. The opinions expressed here do not necessarily reflect those of the U.S. Navy or the Japan Maritime Self-Defense Force.

Image: U.S. Navy (Photo by Mass Communication Spc. 1st Class Jeremy Graham)