Key Takeaways:
- Japan has modified JS Kaga—built as a helicopter carrier—for F-35B STOVL ops and is converting sister ship Izumo. Planned buy: 105 F-35As and 42 F-35Bs. 2024 trials off San Diego with U.S. help; 2026 USMC deck work was planned then canceled. The author says this is not a simple return to Imperial carriers or mere kit upgrades.
- The point, in this reading, is more options if land bases are hit: sea-based air over Japan’s southwest islands, Korea, Taiwan, and China pressure. It also deepens U.S. interoperability. Broader SDF change cited: higher spending, stand-off missiles, subs, drones, space, cyber; nuclear-powered subs discussed, not decided.
- The Kaga name recalls the Pearl Harbor/Midway carrier; the author treats that as geography, not intent, and rejects calling the ships proof of an offensive strategy. Deterrence vs. how China reads Japan’s reach is left open. Challenge: grow flexibility with Washington without looking like a more expansive military role
Japan’s use of JS Kaga for F-35B operations marks an important development in the country’s evolving defense posture and its defense cooperation with the United States (US). Originally designed as a helicopter carrier, Kaga has been modified to support F-35B short-takeoff and vertical-landing aircraft. Japan is converting its sister ship, JS Izumo, to support F-35B operations and plans to acquire 105 F-35As and 42 F-35Bs. These changes are intended to give Japan a growing ability to operate combat aircraft at sea and add a new dimension to its military capabilities. Yet they raise an important question: does Japan’s emerging carrier capability strengthen deterrence and regional security, or does it signal a significant shift in Japan’s military posture that could contribute to greater regional tension?
Neither interpretation fully captures the significance of Kaga. Kaga represents neither a straightforward return to Japan’s carrier past nor merely a technical modernization. It indicates the gradual expansion of Japan’s defensive options through greater maritime airpower, deployment flexibility, alliance interoperability, and resilience.
Expanding Japan’s Defensive Capabilities
Japan is a critical element of Washington’s overall Pacific defense posture, sitting astride the first and second island chains, and hosting some of the most important US military bases in the region. One of the main purposes of Japan’s emerging F-35B maritime air capability is to increase its flexibility in responding to potential threats across its surrounding maritime territory. Aircraft operating from ships can provide options that are not entirely dependent on fixed land-based airfields. This is especially relevant in a region where Japan faces growing military pressure from China, continuing security concerns on the Korean Peninsula, and uncertainty over a potential conflict in the Taiwan Strait, while the protection of its southwestern islands remains an important defense consideration.
The conversion of Kaga shows that Japan’s military modernization extends beyond acquiring new weapons. The ship is being structurally modified to support F-35B operations, including changes to its bow to facilitate the aircraft’s short takeoffs. Japan is therefore developing not only new military equipment, but also the infrastructure, expertise, and operational experience needed to use it effectively. The F-35B gives Japan an option that its land-based F-35A aircraft cannot provide. Operating from suitably modified ships could reduce Japan’s reliance on fixed airfields if conventional airfields were damaged, disrupted, or otherwise unavailable during a conflict.
The development also shows the depth of the defense relationship between Japan and the US. Kaga conducted F-35B shipboard operational testing off the coast of San Diego in 2024, with US support providing an important foundation for Japan’s development of ship-based F-35B operations. The trials included short takeoffs, vertical landings, and other aspects of shipboard operations, providing the Japan Maritime Self-Defense Force (JMSDF) with data and experience needed to develop F-35B operations on the Izumo class. Japanese and US forces have continued to develop their interoperability through bilateral exercises and training. In 2026, Japan and the US planned further deck-landing and takeoff training involving USMC F-35Bs and Kaga, although the exercise was subsequently canceled. The planned training nevertheless demonstrates the extent of operational cooperation supporting Japan’s emerging F-35B capability.
Japan’s Changing Defense Posture
Japan’s development of this capability should be considered against the country’s post-war approach to national defense. For decades, Japanese governments maintained a comparatively restrained conception of the role of military force and avoided describing the large flat-deck vessels operated by the JMSDF as conventional aircraft carriers. The conversion of Kaga and Izumo shows how Japan’s approach to national defense is changing.
The name Kaga has attracted particular attention because it was previously used by an Imperial Japanese Navy (IJN) aircraft carrier that participated in the attack on Pearl Harbor and was later sunk at the Battle of Midway. Some commentators have associated the modern Kaga with Japan’s militarist past. The historical association, however, does not establish a direct institutional or strategic connection between the two ships, nor does the name itself demonstrate offensive intent. Both vessels take their names from Kaga Province, now part of Ishikawa Prefecture. The JMSDF has continued to use historical names for modern vessels, often drawing on geographical features and regional naming traditions.
The more important issue is how Japan intends to use Kaga and Izumo and where these capabilities fit within its evolving defense strategy. Describing Kaga simply as an aircraft carrier also risks obscuring the distinction between possessing a carrier-like capability and adopting an offensive naval strategy. Japan frames the development as part of its effort to strengthen its ability to defend its territory and respond to changes in the regional security environment.
The conversion of Kaga and Izumo is part of a wider transformation in Japan’s defense establishment. In recent years, Tokyo has accelerated the modernization of the Self-Defense Forces (SDF) through increased defense spending, the acquisition of long-range strike capabilities, the development of advanced missile systems, and the expansion of air and maritime capabilities. Japan is strengthening its submarine fleet, upgrading destroyers and fighter aircraft, and investing in uncrewed systems, space capabilities, cyber operations, and other emerging technologies. Tokyo is also considering the option of acquiring nuclear-powered submarines, a major departure from its long-standing reluctance to pursue nuclear-powered naval capabilities, although no decision to do so has been made. The development of stand-off missiles is particularly noteworthy because it expands Japan’s ability to respond to threats at greater distances than was traditionally associated with its defense-oriented posture.
These changes are giving Japan a broader set of military options. The SDF is developing capabilities that can increasingly operate across multiple domains and in conjunction with US forces and other partners. This does not necessarily amount to a transformation into an offensive military power, but it does mark a departure from the more limited conception of national defense that characterized much of the post-war period. Kaga is significant within this process because it combines maritime airpower with greater operational flexibility, survivability, and integration with Japan’s most important security partner.
Deterrence and Regional Tension
Japan’s development of the F-35B capability highlights the tension between strengthening deterrence and avoiding greater regional instability. Expanding its military capabilities is intended to improve its ability to respond to potential threats and raise the costs of aggression. The capability may also contribute to perceptions among regional powers that Japan’s military reach is expanding. Such perceptions are not necessarily an unintended consequence, however, since deterrence depends in part on demonstrating the capacity to respond to potential aggression. Nonetheless, a capability developed for defensive purposes could become part of the strategic competition it is intended to contain.
China is central to this concern, although the precise effect of Japan’s carrier development on Chinese decision-making cannot be known with certainty. Japan’s development of F-35B operations is taking place alongside closer military cooperation with the US and significant changes in the Indo-Pacific security environment. Tokyo and its partners present these developments as measures intended to strengthen deterrence and regional stability, while other governments may interpret them as part of Japan’s shifting military role.
The significance of Kaga extends beyond the question of whether the vessel should be classified as an aircraft carrier. Its development points to a Japanese defense posture that places greater emphasis on operational flexibility, survivability, and reach while continuing to frame these capabilities around territorial defense. How this affects regional stability will depend in part on how Japan employs the capability and how China and other regional powers respond to it.
Kaga is therefore one element of a larger transformation in Japan’s approach to national defense. Its significance lies less in the number of aircraft it can carry than in what it reveals about Japan’s military options and its increasingly integrated defense relationship with the US. For Tokyo, the challenge will be to develop these capabilities while limiting perceptions that Japan is moving towards a more expansive military role.
