The Undersea Dimension: Why Submarines May Decide the Fate of the Taiwan Strait

Surface fleets are photogenic. Carrier strike groups project power in ways that are immediately legible to policymakers, journalists, and the public. But in the confined, shallow, acoustically complex waters of the Western Pacific, the platforms most likely to determine the outcome of a major conflict are the ones you cannot see.

The submarine balance in and around the Taiwan Strait is arguably the single most consequential — and least discussed — factor in Indo-Pacific security. The numbers, the geography, and the physics of undersea warfare tell a story that surface-level force comparisons consistently miss.

The Underwater Order of Battle

Any serious assessment begins with what is in the water. As of early 2026, the relevant submarine fleets break down as follows:

People's Liberation Army Navy (PLAN): Approximately 60 submarines, comprising 6 nuclear-powered attack submarines (SSNs) of the Shang-class (Type 093/093A/093B), 6 nuclear-powered ballistic missile submarines (SSBNs) of the Jin-class (Type 094/094A), and roughly 48 diesel-electric submarines including the improved Kilo-class (Project 636), Yuan-class (Type 039A/B/C), and older Song-class (Type 039/039G) boats. The PLAN submarine force has roughly doubled since 2005.

United States Navy: 49 nuclear-powered attack submarines (SSNs), primarily Los Angeles-class (with ongoing retirement), Virginia-class (the backbone of new construction), and 3 Seawolf-class boats. Of these, approximately 60% are Pacific Fleet assets, though global commitments mean only a fraction are available for any single theater at any given time. The Congressional Budget Office estimated in 2024 that the Navy could surge approximately 12-15 SSNs to the Western Pacific within 30 days of a crisis onset.

Japan Maritime Self-Defense Force (JMSDF): 22 diesel-electric submarines, all Sōryū-class or the newer Taigei-class — among the most capable conventional submarines in the world, equipped with lithium-ion batteries that provide significantly greater submerged endurance than traditional lead-acid designs. Japan plans to increase to 24 boats.

Republic of China Navy (ROCN): 4 submarines — 2 aged Hai Lung-class (Dutch-built, commissioned 1987-88) and 2 World War II-era Guppy-class boats used only for training. Taiwan's Indigenous Defense Submarine (IDS) program aims to deliver 8 new diesel-electric boats, with the first, Hai Kun, launched in September 2023 and undergoing sea trials. Full operational capability for even the lead boat is unlikely before 2027.

Royal Australian Navy: 6 Collins-class diesel-electric submarines, with the AUKUS agreement slating delivery of Virginia-class SSNs beginning in the early 2030s. Australia's current boats are based in Perth — roughly 5,500 km from the Taiwan Strait — but their potential contribution to broader Western Pacific operations is non-trivial.

The Physics of the Problem

Raw numbers are necessary but insufficient. Submarine warfare is fundamentally shaped by physics — specifically, the physics of sound propagation in water. And the waters around Taiwan present a uniquely challenging acoustic environment.

The Taiwan Strait itself is shallow — averaging roughly 60 meters in depth, with some areas as shallow as 20 meters. For submarine operations, this is extraordinarily constrained. Nuclear submarines, which are generally larger and optimized for deep-water operations, face significant limitations in such environments. The US Navy's Virginia-class boats, at approximately 7,900 tonnes submerged displacement and 115 meters in length, can operate in shallow water but lose many of their deep-water acoustic advantages.

Conversely, smaller diesel-electric submarines — particularly modern designs with air-independent propulsion (AIP) or lithium-ion batteries — are optimized for exactly these conditions. Running on battery power, a modern conventional submarine is essentially silent. The JMSDF's Taigei-class boats, at roughly 3,000 tonnes, can operate in the shallow littoral waters of the East China Sea with a noise signature that approaches the ambient ocean background.

This creates an asymmetry that favors the defender in ways that are often underappreciated:

• Detection ranges compress dramatically in shallow water due to bottom reflections, surface duct effects, and biological noise. A submarine that might be detected at 50+ km in deep water may be undetectable beyond 5-10 km in the Taiwan Strait.
• The thermocline layer — the temperature gradient that affects sonar propagation — is less stable in shallow seas, creating unpredictable acoustic conditions that make systematic search patterns unreliable.
• High shipping density in the Taiwan Strait (one of the world's busiest waterways) creates persistent background noise that masks submarine signatures, particularly for slow-moving diesel-electric boats.

The net effect: the Taiwan Strait and its approaches are, from a submarine warfare perspective, a defender's paradise and an attacker's nightmare.

The PLAN Submarine Threat: Scale vs. Quality

China's submarine fleet is large and growing. But aggregate numbers obscure critical qualitative distinctions that matter enormously in combat.

The PLAN's nuclear submarine program has been its most persistent capability gap. The Shang-class SSNs, while a significant improvement over the elderly Han-class boats they replaced, remain substantially noisier than their US counterparts. The Office of Naval Intelligence (ONI) assessed in its 2023 report that the Type 093B's acoustic signature is roughly comparable to early Los Angeles-class boats from the 1980s — a generation behind the Virginia-class in quieting technology.

This matters because nuclear submarines are the platforms China would need for sustained operations east of Taiwan — in the deep waters of the Philippine Sea, where they might attempt to interdict US reinforcements or establish a defensive perimeter. In deep water, against Virginia-class and Seawolf-class opponents with decades of acoustic advantage, PLAN SSNs face a significant qualitative deficit.

China's conventional submarine force is more competitive. The Yuan-class boats incorporate AIP systems (likely Stirling engines) that allow extended submerged operations without snorkeling. These boats are well-suited to operations in the shallow waters west of Taiwan and in the strait itself. However, they share the fundamental limitation of all conventional submarines: limited range and endurance compared to nuclear boats, confining their effective operational radius to roughly 1,500-2,000 km from home port.

The PLAN has also invested heavily in submarine-launched anti-ship cruise missiles (ASCMs). The YJ-18, deployed on both Shang-class SSNs and Yuan-class SSKs, provides a subsonic cruise phase followed by a supersonic terminal sprint — a challenging target for shipboard defenses. A coordinated salvo from multiple submarines could saturate a surface group's defensive systems.

The Coalition Undersea Advantage

If the PLAN's submarine force presents a genuine threat, the coalition undersea picture presents an equally genuine — and arguably more decisive — advantage.

The US submarine force remains the most capable in the world by a wide margin. Virginia-class SSNs combine exceptional quieting, advanced sonar suites (including the Large Aperture Bow array and lightweight wide-aperture arrays), and heavy weapons loadouts (37 torpedo-tube-launched weapons plus, on Block V boats, the Virginia Payload Module with 28 additional Tomahawk cells). In a contested environment where surface ships and aircraft face dense anti-access threats, submarines may be the only platforms capable of operating with relative impunity.

The CSIS wargame series (2022-2023) consistently found that US submarines accounted for the majority of PLAN surface combatant and amphibious ship losses in Taiwan defense scenarios — in some iterations, sinking more tonnage than all other coalition platforms combined. The logic is straightforward: in a threat environment saturated with anti-ship ballistic missiles and long-range cruise missiles, submarines are the most survivable offensive platforms available.

Japan's contribution is equally significant in context. The JMSDF's 22 submarines, operating in their home waters with intimate knowledge of local acoustic conditions, represent a force multiplier that is difficult to quantify but impossible to ignore. Japanese submarines conducting barrier operations in the Miyako Strait or the Tokara Strait could effectively bottle up PLAN surface forces attempting to break into the Pacific — or destroy them in the attempt.

A 2024 RAND study on undersea warfare in the Western Pacific concluded that coalition submarine forces, properly positioned, could impose attrition rates of 15-25% on a PLAN amphibious fleet during a strait crossing — before a single surface combatant or aircraft engaged. For an operation requiring the transport of an estimated 300,000-500,000 troops and their equipment, those losses would be catastrophic.

Taiwan's Submarine Gap — and Why It Matters

Taiwan's own submarine force represents perhaps the most significant capability gap in its defense posture. With only 2 operationally relevant boats (the Hai Lung-class), Taiwan lacks the undersea capacity to independently threaten a crossing force at scale.

The Indigenous Defense Submarine program is a strategic imperative for precisely this reason. Even a modest force of 8-10 modern diesel-electric submarines, operating in the shallow waters of the strait where they hold maximum advantage, could impose disproportionate costs on an amphibious operation. A submarine does not need to sink a troop transport to be effective — its mere possible presence forces an adversary to divert enormous anti-submarine warfare (ASW) resources, slowing operations, consuming assets, and introducing uncertainty that degrades operational planning.

This is the concept submariners call the "fleet in being" effect: a submarine force that exists and might be anywhere forces an opponent to assume it could be everywhere. During the Falklands War, a single British nuclear submarine — HMS Conqueror — sank the Argentine cruiser General Belgrano and effectively confined the entire Argentine surface navy to port for the remainder of the conflict. The lesson has not been lost on defense planners in Taipei.

The challenge is time. Submarine construction is among the most technically demanding industrial processes in defense manufacturing. Taiwan's shipbuilding industry, while capable, has never produced a submarine. The IDS program faces the dual challenge of mastering new technology while operating under persistent Chinese diplomatic pressure that has historically prevented Taiwan from acquiring submarines from foreign builders.

The Anti-Submarine Warfare Problem

If submarine warfare favors the defender, then anti-submarine warfare (ASW) — the art of finding and killing submarines — represents the attacker's critical requirement. And ASW is extraordinarily difficult.

The fundamental challenge is mathematical. The ocean is vast, submarines are small, and the search problem is three-dimensional. A single Virginia-class submarine operating in a patrol area of 10,000 square nautical miles presents a search problem that can occupy dozens of ASW platforms for days without result.

China has invested significantly in ASW capabilities — maritime patrol aircraft (the KQ-200, analogous to the US P-8 Poseidon), towed-array sonar systems on surface combatants, ASW helicopters (Z-20F), and reportedly a network of fixed undersea sensors in the South China Sea and approaches to the First Island Chain.

However, ASW proficiency is not primarily a hardware problem. It is an institutional and experiential one. Effective ASW requires decades of accumulated operational experience — the tacit knowledge of how to interpret ambiguous sonar contacts, how to manage search patterns in varying acoustic conditions, how to coordinate multiple platforms in a dynamic undersea environment. The US Navy and the JMSDF have conducted ASW operations continuously since the Cold War. The PLAN has no comparable operational heritage.

A 2025 assessment by the International Institute for Strategic Studies noted that despite significant hardware investments, the PLAN's ASW capability remains its most significant operational weakness — a gap that cannot be closed by procurement alone and likely requires another decade or more of intensive training and operational development.

Mines: The Silent Multiplier

No discussion of undersea warfare in the Taiwan Strait is complete without addressing naval mines — arguably the most cost-effective weapons in any maritime inventory.

The Taiwan Strait's shallow depth, confined geography, and limited number of suitable landing beaches make it ideally suited for defensive mining. Taiwan maintains a stockpile of several thousand naval mines, including modern influence mines that can be programmed to activate based on acoustic, magnetic, or pressure signatures — allowing them to selectively target specific vessel types.

Submarines are ideal mine-delivery platforms, capable of covert deployment in waters that surface minelayers cannot safely access. A small number of submarines conducting mine-laying operations in the approaches to Taiwan's western beaches could create minefields that would require days or weeks of mine countermeasure operations to clear — time that an amphibious force simply does not have if it hopes to achieve the element of surprise.

Historical precedent is instructive. During the Korean War, a North Korean minefield of approximately 3,000 contact mines delayed the Wonsan amphibious landing by two weeks. Modern influence mines are orders of magnitude more difficult to sweep. The US Navy's own mine countermeasure capacity is widely acknowledged as inadequate — a deficiency that applies equally to the PLAN, whose mine countermeasure fleet is small relative to the scale of the potential mining problem.

The Decisive Domain

Surface naval battles are dramatic. Air campaigns are visible. Cyber operations make headlines. But the undersea domain may be where a Taiwan Strait conflict is actually decided — quietly, invisibly, in the dark waters between the seabed and the thermocline.

The structural dynamics favor defense. Shallow waters compress detection ranges and neutralize the deep-water advantages of nuclear submarines. Diesel-electric boats operating on battery power are functionally undetectable. Mines transform navigable waters into kill zones. And the geographic chokepoints of the First Island Chain — the straits that any PLAN force must transit to operate in the Pacific — are ideal environments for submarine ambush.

For an amphibious invasion force — the most vulnerable of all naval formations, laden with slow, high-value transports that cannot maneuver evasively — the undersea threat is existential. A single torpedo hit on a troop transport carrying 1,000 soldiers is not merely a tactical setback. It is a strategic event — one that would be repeated, in the models, dozens of times during a strait crossing against a competent submarine defense.

This is why the submarine balance matters more than the surface fleet count, more than the missile inventory, perhaps more than any other single factor. It is also why investments in undersea capability — whether Taiwan's IDS program, Japan's Taigei-class expansion, Australia's AUKUS submarine pathway, or the US Virginia-class production rate — represent some of the highest-leverage defense spending in the Indo-Pacific.

The platform that decides the fate of the Taiwan Strait may never be photographed, never appear on the evening news, never be discussed in a presidential debate. It will be 60 meters beneath the surface, running silent, waiting.

Further Reading

• Why Taiwan Is Nearly Impossible to Invade: A Military Analysis
• The Japan Factor: Why Tokyo Is the Linchpin in Any Taiwan Contingency
• Taiwan's Porcupine Strategy: The Shift to Asymmetric Defense