Silent Alliance: Russia and China’s First Joint Submarine Patrol in Sea of Japan Shocks Indo-Pacific

(DEFENCE SECURITY ASIA) — The world has just witnessed a dangerous new chapter in maritime security.

For the first time, Russia and China openly conducted a joint submarine patrol in the Sea of Japan, signalling that their naval cooperation has moved from the surface into the silent and most secretive arena of undersea warfare.

This unprecedented maneuver followed the Maritime Interaction/Joint Sea 2025 exercise, a combined war game in Vladivostok and the Sea of Japan, involving destroyers, corvettes, fleet oilers, submarine rescue ships, and submarines from both navies.

At the heart of the patrol was Russia’s Volkhov (B-603), a Project 636.3 Improved Kilo-class diesel-electric submarine, supported by the Pacific Fleet’s corvette RFS Gromkiy (335).

Japan’s Maritime Self-Defense Force (JMSDF) reported spotting the Russian submarine group twice—on August 14 and August 20—as it transited the Tsushima Strait, one of Japan’s most vital maritime chokepoints.

Volkhov was detected sailing on the surface, a deliberate signal that its presence was meant to be seen and noted by Tokyo and Washington.

The JMSDF also tracked PLAN destroyer CNS Urumqi (118) and submarine rescue ship CNS Xihu (841) on August 13, while the PLAN’s Great Wall 210 submarine was likely submerged, evading detection as it slipped past Japanese sensors.

This combined undersea patrol, immediately following surface drills involving Russian destroyer RFS Admiral Tributs (564), submarine rescue ship Igor Belousov, and PLAN destroyer CNS Shaoxing (134), marks a dramatic escalation in the depth and sophistication of Sino-Russian military cooperation.

Why This Patrol Is Different

Unlike previous joint naval operations confined to surface ship manoeuvres or aerial patrols, this patrol deliberately crossed into the subsurface domain.

Submarines remain the crown jewels of naval warfare, designed to operate undetected, carrying missiles and torpedoes that can cripple fleets or strike targets ashore.

By combining forces underwater, Moscow and Beijing are quietly laying the foundation for a new undersea alignment in the Indo-Pacific, one that will directly challenge the anti-submarine warfare (ASW) dominance of the United States, Japan, and their allies.

Several factors stand out.

First, the patrol demonstrated binational coordination in submerged operations, traditionally one of the most sensitive aspects of naval warfare, requiring common procedures, recognition signals, and acoustic deconfliction.

Second, the patrol took place right beside Japan’s most critical maritime arteries—the Tsushima and La Pérouse Straits—waters that Tokyo considers its front line of defence.

Third, the operation revealed a willingness by both powers to accept higher political and military risks by entering waters where allied ASW networks are strongest, suggesting confidence in their submarines’ survivability and stealth.

Fourth, it underlined a strategic shift in signalling: whereas past exercises sought visibility for deterrence, this patrol emphasized deniability and ambiguity, the very hallmarks of grey-zone submarine warfare.

Fifth, by coordinating submerged patrols, Russia and China have set the stage for future integrated deterrence missions, where their submarines could one day protect each other’s ballistic missile submarines (SSBNs) in bastion zones during a regional crisis.

“This is more than a training exercise—it is a baptism of trust,” observed one Asian naval strategist, “because you do not risk operating submarines together unless you are building something much deeper: interoperability and shared deterrence.”

Sixth, analysts increasingly interpret the patrol as a dress rehearsal for escalation management, with both navies testing the threshold between routine competition and warlike posturing, gauging allied reactions in real time.

“This was not a stunt,” warned a retired Japanese admiral, “it was a measured probe to see how quickly the JMSDF and U.S. Navy could detect, classify, and track a mixed Sino-Russian undersea presence.”

Seventh, the patrol’s deliberate proximity to Japan not only rattles Tokyo’s strategic calculus but also forces Washington to allocate more ASW assets northward, indirectly giving China greater freedom of manoeuvre in flashpoints such as Taiwan and the South China Sea.

Eighth, by openly showcasing submarine cooperation, Moscow and Beijing are declaring that their partnership has evolved beyond political theatre into a functional military coalition capable of contesting U.S. and allied dominance beneath the waves.

“This patrol should be read as a strategic milestone,” concluded one European defence analyst, “because the undersea domain has long been America’s ace card. The fact that Russia and China are now willing to challenge it jointly is a profound warning to the Indo-Pacific alliance structure.”

Military Significance: What Both Navies Gain

Submarine cooperation at this level is far more than symbolic.

It forces both navies to test underwater interoperability, including coordination of patrol areas, acoustic recognition signals, and water-space management (WSM) rules.

This is a rehearsal for combined operations in crisis, where Chinese and Russian submarines may one day operate in the same theatre, requiring seamless communication and rescue protocols.

For Russia, the patrol allowed its Improved Kilo-class submarine to work alongside a modern Chinese Yuan-class (Type 039A/B/C) submarine, believed to be the Great Wall 210.

The Improved Kilo is prized for its stealth characteristics, extremely low acoustic signature, and the ability to launch the Kalibr-PL family of cruise missiles, with ranges up to 2,500 km for land-attack variants.

This makes Volkhov not just an ambush predator in littoral waters but also a strategic strike platform capable of hitting military bases, ports, and infrastructure deep inland.

The Chinese Yuan-class brings complementary capabilities.

Its air-independent propulsion (AIP) system allows it to remain submerged for extended periods, reducing the vulnerability of snorkeling that betrays older diesel-electric boats.

Armed with the YJ-18 cruise missile, which cruises at subsonic speed before sprinting at supersonic velocity in its terminal phase, the Yuan is a nightmare for carrier strike groups and surface fleets.

Together, the Russian Kilo and the Chinese Yuan create a layered strike profile—with one platform optimized for stealthy missile launches and the other for long-endurance silent patrols.

This synergy complicates any defender’s targeting solution and stretches thin the sonar networks and patrol aircraft of Japan and the United States.

Stress-Testing Japan’s Anti-Submarine Warfare Shield

The Sea of Japan is perhaps the worst location for Tokyo to see this cooperation emerge.

Japan maintains one of the most advanced ASW ecosystems in the world, with Soryu and Taigei-class submarines, Kawasaki P-1 maritime patrol aircraft, and dense seabed sensor networks.

But joint Sino-Russian submarine patrols in this semi-enclosed basin present Tokyo with a multi-front challenge.

Each patrol not only tests the reaction time of Japan’s ASW forces but also builds up a valuable library of signals intelligence on Japanese and American tracking methods.

For China, this is especially critical.

The PLAN’s diesel-electric submarines rarely get the chance to operate in such contested waters against such capable adversaries without the cover of Russian cooperation.

By operating under Moscow’s shadow, Beijing gains priceless insights into how Japan and the U.S. conduct real-world ASW.

The Role of Submarine Rescue Ships

Another overlooked but vital element was the participation of submarine rescue vessels Igor Belousov (Russia) and Xihu (China).

This detail signals something profound.

Both navies now feel confident enough to push their submarines into confined and heavily monitored waters because they have mutual assurance of rescue and recovery.

Submarine accidents are politically explosive, but by practicing combined rescue doctrine, Moscow and Beijing lower the risk of operational embarrassment.

This emboldens commanders to conduct riskier maneuvers closer to enemy coastlines, confident that if an accident occurs, recovery is possible.

Geopolitical Signaling Across the Region

The joint submarine patrol was not only about training.

It was about sending messages to every capital in the region.

To Tokyo, it signaled that Japan must now prepare for undersea pressure from two nuclear powers simultaneously.

To Washington, it served as a warning that Sino-Russian cooperation has expanded beyond surface demonstrations to the silent domain of submarines, directly challenging U.S. undersea dominance.

To Seoul, the patrol reminded South Korea that the Sea of Japan is no longer just about deterring North Korean submarines. It is now a contested battlespace where Chinese and Russian signatures must be accounted for in ROK Navy operations.

To Taipei and Manila, the patrol demonstrated bandwidth diversion. If U.S. and Japanese ASW assets are forced to focus north, the PLAN has more freedom of maneuver around Taiwan, the South China Sea, and the Luzon Strait.

Tactical Shifts in the Undersea Game

Joint patrols also pave the way for multi-axis submarine tactics.

Two dissimilar submarines can create dilemmas for defenders.

One vessel can act as the decoy, attracting sonobuoys and patrol aircraft, while the other maneuvers silently into strike position.

This forces ASW forces to split their efforts and risks saturation of their defenses.

Russia brings to the table its Kalibr employment doctrine, wake-homing torpedoes, and electronic warfare tactics for noisy littorals.

China contributes its AIP endurance, modern combat systems, and YJ-18 missile tactics.

The result is a dangerous cross-pollination of tactics, techniques, and procedures (TTPs) that raise the survivability and lethality of both fleets.

Strategic Impact: Bastions, Deterrence, and Grey-Zone Warfare

At a higher level, the patrol links directly to the strategic bastion concepts of both navies.

For Russia, the Sea of Okhotsk serves as the bastion for its ballistic missile submarines (SSBNs).

For China, the Bohai Gulf and South China Sea are its chosen sanctuaries for SSBN patrols.

By building habits of cooperation in SSK operations, Moscow and Beijing lay the groundwork for future joint protection of SSBN bastions, complicating allied strategic ASW efforts.

Beyond deterrence, the patrol highlights vulnerabilities in undersea critical infrastructure.

Japan’s submarine cables, seabed surveillance networks, and energy pipelines are increasingly within reach of Sino-Russian submarines.

Even without direct attack, the mere presence of combined submarines near these assets constitutes a form of grey-zone leverage, forcing Japan to devote resources to monitoring and protection.

Likely Allied Countermeasures

This patrol will not go unanswered.

Expect the United States, Japan, and South Korea to intensify ASW patrols, deploying more P-8A Poseidon and P-1 aircraft, laying additional fixed arrays, and expanding seabed monitoring.

Submarine-on-submarine shadowing will become routine, with Japanese Taigei and U.S. Virginia-class boats trailing Sino-Russian patrols from the moment they leave port.

Allies will harden seabed infrastructure with redundant cables, faster repair capacity, and autonomous monitoring systems to mitigate grey-zone threats.

Trilateral ASW drills will increasingly focus on multi-contact prosecutions, rehearsing how to handle dissimilar submarine threats in the same battlespace.

In parallel, Washington and Tokyo are likely to accelerate investments in next-generation passive sonar networks and AI-driven acoustic processing, designed to detect ultra-quiet AIP and Kilo-class signatures in noisy littoral waters.

South Korea may also fast-track the deployment of its KSS-III Batch II submarines, fitted with advanced sonar suites and vertical launch cells, to add both offensive reach and counter-detection capabilities.

Ultimately, the allies will treat the Sea of Japan as a permanent contested battlespace, requiring round-the-clock surveillance, layered defences, and coordinated ASW doctrines to blunt the momentum of Sino-Russian undersea cooperation.

The Bottom Line

The first Sino-Russian joint submarine patrol in the Sea of Japan is not a one-off stunt.

It represents the silent alignment of two powers willing to cooperate beneath the waves, in the most decisive domain of modern naval warfare.

For Japan and its allies, this is a wake-up call that the balance of power beneath the sea is shifting.

A Kalibr-armed Kilo working in tandem with an AIP-quiet Yuan presents a lethal combination that stretches ASW defences and undermines confidence in maritime chokepoint security.

Strategically, the patrol pushes the region toward a future where the contest is not only about carriers and missiles above the surface but also about cables, sensors, and diesel-electric submarines below it.

For Tokyo, Washington, and Seoul, the challenge is clear: respond not with alarmism, but with persistence, superior sensors, hardened seabed assets, and relentless ASW presence.

Because in the new silent war under the waves, acoustic superiority will decide the fate of fleets, trade, and deterrence in the Indo-Pacific.

More critically, the joint patrol signals that Moscow and Beijing are prepared to erode America’s traditional dominance in the undersea domain, long regarded as the ultimate insurance policy for U.S. power projection.

The move also underscores a strategic convergence: Russia brings experience in submarine warfare honed against NATO in the North Atlantic, while China brings industrial capacity and growing undersea numbers, together forming a formidable blend of quantity and quality.

If such patrols become routine, they could eventually normalize a two-power undersea presence in Northeast Asia, forcing the U.S.–Japan–ROK alliance to permanently adjust force posture and budgets to sustain round-the-clock submarine tracking.

— DEFENCE SECURITY ASIA