Japan’s hypersonic subs bring it closer to the nuclear edge

Japan’s push for submarine-launched hypersonic weapons marks a major step toward a more survivable counterstrike posture — one that could eventually give it the foundations of a latent sea-based nuclear deterrent.

Japan’s Ministry of Defense (MoD) officially included plans to develop underwater-launched hypersonic missiles in the fiscal year 2027 budget request, intended to strengthen its standoff “counterstrike capability” against adversary bases, multiple media outlets reported.

A MOD rendering shows a submarine with two or three vertical launch system (VLS) tubes, each carrying seven or eight solid-fuel boost-glide Hyper Velocity Gliding Projectile (HVGP) missiles.

A boost-glide weapon uses a large rocket booster to reach the upper atmosphere, with the glider coasting to the target at Mach 5 and faster. While practical, the large booster limits the missile count, and each maneuver costs energy with no speed recovery.

Later versions of the HVGP are designed for longer range, with the Enhanced Capability Version Block 2A having a 2,000-kilometer range and the Block 2B planned for 3,000 kilometers.

Japan is also developing scramjet-powered hypersonic cruise missile (HCM) technology for future underwater launch.

Unlike a boost-glide weapon, an HCM uses a booster to accelerate to the speed at which its scramjet can operate; the engine then ingests supersonic airflow and sustains thrust in flight. That could enable a more compact missile, but the technology remains complex and immature.

Alongside crewed platforms, Japan is researching launch capabilities to arm modular unmanned underwater vehicles (UUVs) with long-range strike weapons, thereby supplementing Japan’s smaller submarine fleet.

Deploying these stealthy, submerged strike systems would create a persistent, hard-to-detect threat capable of holding Chinese naval combatants and amphibious fleets at risk across the First Island Chain during a regional conflict.

The program, included in Japan’s broader FY2027 defense budget request, signals a shift toward more survivable maritime strike platforms.

The key question is whether Japan’s submarine-launched hypersonic weapons can overcome the operational and political limits of its counterstrike posture while also laying the foundations for a more survivable—and potentially nuclear-capable—sea-based deterrent.

That survivability matters most along chokepoints such as the Miyako Strait, where Chinese naval forces regularly pass through the First Island Chain into the open Pacific. In a Taiwan contingency, access through the strait could support Chinese efforts to enforce a blockade, threaten Japan’s sea lanes of communication (SLOCs), and complicate US intervention.

PLAN surface combatants, especially carrier strike groups (CSGs), crossing the Miyako Strait would likely have layered defenses: outer layers of forward-deployed fighters and submarines, with carriers protected by frigates, destroyers, and cruisers with long-range air and missile defense systems.

Soft-kill systems such as electronic warfare, chaff and flares, backed by hard-kill close-in weapons systems (CIWS), provide the final layer of defense.

Legacy anti-ship missile systems such as Japan’s Type 88 may no longer be up to the task against contemporary ship defenses. Like the long-serving US Harpoon, it flies at a low subsonic speed with a sea-skimming flight profile and a relatively short range of 100 kilometers.

However, its limited range, subsonic speed, and lack of stealth features could mean it is no longer effective against contemporary air defenses, prompting missile design to prioritize stealth and speed. Japan appears to be developing both types of missiles to penetrate such defenses, including the stealthy Type 12 anti-ship missile and its HVGP program.

A submarine-launched hypersonic weapon would also make Japan’s counterstrike force harder to suppress than strike assets concentrated at exposed air bases, fixed land sites or large surface combatants.

For instance, a January 2025 Hudson Institute report notes that Japan faces a severe strategic vulnerability due to a critical shortage of modern hardened aircraft shelters (HASs), leaving its aircraft vulnerable to destruction on the ground.

The report states that while Japan retains 140 Cold War-era shelters concentrated in central and northern regions, the country has built only two new HASs over the past decade.

Japan may also lack sufficient interceptor missiles to withstand a saturation attack. A June 2026 Hudson Institute report shows that Japan faces a severe interceptor shortage underscored by a zero-procurement request for Patriot Advanced Capability-3 Missile Segment Enhancement (PAC-3 MSE) interceptors in its FY2026 budget, leaving its air defense architecture critically vulnerable to expanding regional missile arsenals.

According to the report, simulations of a Taiwan contingency reveal that Japan’s PAC-3 MSE inventories would be exhausted within days under high-intensity combat against saturation strikes.

While Japan has integrated US-made Tomahawk cruise missiles on its destroyers, large surface warships may be vulnerable to missile attacks, unmanned surface vehicles (USVs), and submarines.

But survivability alone does not make a credible counterstrike force: Japan still needs intelligence, surveillance, and reconnaissance (ISR) to find targets, doctrine to govern their employment, communications to signal intent, and political will to use the weapons.

Japan’s space-based ISR capabilities remain limited, with only nine ISR satellites in service as of March 2026. This could leave Japan dependent on the US as it builds its indigenous ISR capabilities, raising concerns about strategic autonomy.

In terms of doctrine, Japan’s counterstrike capabilities present a dilemma: a counterstrike after an adversary missile has been launched is useless; attacking too early risks unintended escalation.

Political sensitivities in China and South Korea could pose an obstacle to Japan’s strategic communications, forcing Japan to present inherently offensive weapons as defensive. Longstanding pacifist norms could work against Japan’s current government’s push to acquire counterstrike capabilities, limiting the scope of their development and the timing of their use.

Those same investments could also deepen Japan’s nuclear latency by creating the infrastructure for a survivable sea-based delivery option. South Korea offers a regional analog: its KSS-III Dosan Ahn Changho-class conventional submarines carry six conventionally armed submarine-launched ballistic missiles (SLBMs) for land attack.

South Korea’s SLBMs are conventionally armed, underscoring that submarine-launched missiles do not by themselves imply a nuclear mission. Their significance for Japan is different: pairing a survivable submarine with long-range vertical-launch weapons creates part of the delivery architecture that could support a nuclear deterrent if a future government ever made that political choice.

Japan’s leadership may already be testing the political boundaries of the nuclear debate: Prime Minister Sanae Takaichi has declined to reaffirm the country’s longstanding non-nuclear principles, while Defense Minister Shinjiro Koizumi has called for more open discussion of nuclear weapons.

Japan already possesses substantial fissile-material stockpiles and an advanced civilian nuclear and scientific base, but it lacks a submarine-based vertical-launch architecture comparable to South Korea’s.

Adding VLS-equipped submarines and long-range hypersonic weapons would therefore narrow a key delivery-system gap without, by itself, constituting a decision to pursue nuclear weapons.

If Japan can pair survivable undersea strike with independent ISR, credible doctrine and political resolve, the program could strengthen conventional deterrence while lowering the technical barrier to a future sea-based nuclear option should Japan’s strategic choices change.