U.S. Japan and South Korea Submarines Rehearse Undersea Combat as China Expands Pacific Fleet.

The U.S. Navy, Japan Maritime Self-Defense Force, and Republic of Korea Navy conducted direct anti-submarine warfare training in the Western Pacific, tracking one another, maneuvering at close range, and simulating undersea engagements. The exercise strengthens allied ability to detect and counter hostile submarines in waters critical to operations beyond the first island chain.

A Royal Australian Air Force P-8A Poseidon added airborne surveillance and targeting support, creating a four-nation undersea warfare scenario combining submarines with long-range maritime patrol aviation. The training reflects growing allied emphasis on coordinated ASW to complicate the operations of China’s expanding submarine force and improve deterrence across the Western Pacific.

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U.S. Navy submarine underway during undersea warfare operations, illustrating U.S.-Japan-South Korea anti-submarine cooperation in the Western Pacific. (Picture source: U.S. Navy)

The U.S. Navy Commander, Submarine Group Seven, disclosed the August 25–27, 2026 exercise from Yokosuka, Japan, on September 4. The three submarines also practiced joint water-space management, while a Royal Australian Navy liaison officer worked from Commander Task Force 74’s Theater Undersea Warfare Commander Maritime Operations Center, linking the tactical activity to U.S. theater-level undersea command and control.

The most important feature was not the number of forces involved but the fact that submarines from the United States, Japan, and South Korea were placed into the same tactical problem. Multinational surface and air exercises are common in the Indo-Pacific, but submarine cooperation is significantly more sensitive because it involves acoustic signatures, covert operating procedures, restricted communications, and strict control of underwater operating areas. The U.S. Navy said the submarines conducted mutual tracking and simulated engagements rather than simply operating alongside one another, meaning each crew had to detect and classify another submarine, maintain contact, maneuver without revealing unnecessary acoustic information, and react to the possibility of being tracked in return. Those are core wartime tasks in any high-end submarine confrontation.

Joint water-space management made the exercise equally relevant at the operational level. In a Western Pacific conflict, U.S., Japanese, and South Korean submarines could operate in adjacent areas while surface combatants and maritime patrol aircraft search for hostile submarines above them. Coordinating those operating spaces is essential to prevent friendly interference, preserve freedom of maneuver, and avoid confusing allied submarines with hostile contacts. Australia’s P-8A Poseidon added a fourth national layer to that undersea problem by bringing a long-range aircraft capable of deploying sonobuoys, processing acoustic contacts, and rapidly repositioning between distant search areas. Used together with allied submarines, the P-8A can create a layered search environment in which an opposing submarine must account simultaneously for airborne acoustic sensors and submerged threats whose locations remain unknown.

That combination is directly relevant to the China submarine threat. The People’s Liberation Army Navy continues to modernize both its nuclear-powered and conventional submarine forces while expanding operations into the Philippine Sea and wider Western Pacific. For U.S. and allied planners, the challenge is not simply detecting Chinese submarines close to the Chinese coast, but maintaining awareness as they move through or beyond the first island chain into much larger areas of the Pacific. A Chinese submarine operating east of that chain would face a harder problem if American, Japanese, and South Korean submarines could work from compatible water-space plans while Australian P-8A crews added long-range airborne surveillance. Instead of preparing to evade one national anti-submarine force, a submarine commander would have to account for several allied sensor networks and multiple potential tracking units distributed across a very large maritime area.

This is where the August exercise becomes more important than a conventional Japan-South Korea naval drill. The United States has separate alliances with Tokyo and Seoul and exceptionally close defense ties with Australia, but the exercise connected elements of all four relationships inside a single undersea-warfare problem. The three submarines formed the covert tactical layer, the P-8A provided rapid airborne search capability and CTF-74 supplied theater-level coordination from Yokosuka. For China, that type of integration creates uncertainty, which is itself a form of deterrence. A submarine moving into the Western Pacific would have greater difficulty predicting which allied force might first detect it, which unit would maintain contact or whether tracking responsibility could shift from an aircraft to a submarine from another country.

Japan is particularly important in this geography because the Japanese archipelago forms a major part of the first island chain and sits across several routes connecting Chinese coastal waters with the broader Pacific. Japanese submarines operating in concert with U.S. forces can help monitor those approaches, while South Korean submarines add another advanced conventional undersea force that can support operations farther north around the Korean Peninsula and surrounding seas. South Korea’s participation therefore broadens what has traditionally been a heavily U.S.-Japan-centered undersea relationship, and closer coordination among the three submarine forces could reduce the opportunities for hostile submarines to exploit seams between separate allied operating areas during a crisis.

The exercise also demonstrates why U.S. Navy anti-submarine warfare increasingly depends on coalition integration rather than American forces alone. The Western Pacific is too large for one navy to maintain continuous submarine coverage across every important route, chokepoint and operating area. Linking allied submarines with maritime patrol aircraft spreads search, tracking, and contact-maintenance responsibilities across a wider force. That distributed approach becomes especially important once a submarine moves into the open Pacific, where detecting a submerged contact is only the first step and maintaining that contact over time requires aircraft availability, acoustic coverage and forces positioned to take over the track if another unit must leave.

The U.S. Navy did not identify the three submarines, their classes or the exact operating area, and it did not state that China was the exercise’s simulated adversary. Those limitations are important, but they do not change the strategic context in which the training took place: the Western Pacific is the central maritime theater for U.S. deterrence against China, and undersea warfare is one of the areas in which both sides are investing heavily. The secrecy surrounding the participating submarines reinforces why the exercise is unusual, because naval forces closely protect submarine acoustic characteristics, sensor performance and tactical behavior, making direct multinational submarine training a stronger indicator of operational trust than large surface formations or ceremonial exercises.

The August 2026 drill therefore marks a notable step from alliance presence toward coalition submarine warfare. Three allied submarines were not simply deployed to the same region; they tracked one another, conducted simulated engagements, and practiced the water-space coordination required to fight in the same undersea environment. Adding an Australian P-8 Poseidon and U.S. theater-level undersea command architecture makes that development strategically more significant, because Chinese submarines moving beyond the first island chain could increasingly face not one national ASW force, but a connected network of allied submarines, aircraft and command centers capable of detecting, tracking and pressuring them across a much larger battlespace.

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Written by Alain Servaes – Chief Editor, Army Recognition Group
Alain Servaes is a former infantry non-commissioned officer and the founder of Army Recognition. With over 20 years in defense journalism, he provides expert analysis on military equipment, NATO operations, and the global defense industry.