In late July, Chinese state television aired footage of an elite PLA Air Force brigade patrolling both the Bashi Channel and the Tsushima/Korea Strait. The segment strongly implied that J-20 stealth fighters had transited the narrow waterway between Korea and Japan. Neither Tokyo nor Seoul publicly confirmed an intercept or even a detection. Beijing’s intent was unmistakable: to show that its most advanced aircraft could fly through one of the most heavily monitored chokepoints in Asia without consequence.
The geography makes this significant. The Korea Strait connects the East China Sea to the Sea of Japan, within reach of key U.S. and Japanese bases critical to any Taiwan contingency. Some commentary even claimed the J-20 “evaded THAAD,” but this was propaganda rather than proof. The THAAD radar is optimized for ballistic missile defense, not stealth aircraft tracking. What matters is that a Chinese stealth patrol in this corridor highlights the challenge facing U.S.–Japan–ROK integrated air and missile defense.
The Dual Contingency Risk
U.S. and allied planners increasingly treat a dual contingency—China attacking Taiwan while North Korea exploits the crisis—as a central assumption, not a remote scenario. In such a case, the ability of China’s J-20s to operate undetected through the Korea Strait would create disproportionate problems for both deterrence and warfighting. The danger is not simply that a single stealth fighter slipped past radar; it is that China could normalize this flight path, opening a vector for coordinated pressure against both Japan and South Korea at precisely the moment the United States would be forced to divide its attention.
In a Taiwan contingency, allied forces would rely heavily on AWACS, tankers, and maritime patrol aircraft orbiting near the Korea Strait and Sea of Japan to knit together the regional air picture and to keep open the logistical arteries feeding into the peninsula. If J-20s can maneuver through this corridor unseen, they would be positioned to threaten these high-value assets with long-range air-to-air missiles such as the PL-15. Even the perception that stealth fighters could hold allied AWACS or tankers at risk would force them to orbit farther from the battlespace, degrading surveillance coverage, shortening loiter time, and slowing response cycles. In a fast-moving air campaign, that could mean the difference between timely intercepts and exploitable gaps.
The implications stretch further. A successful undetected transit undermines confidence in the credibility of emerging U.S.–Japan–ROK air and missile defense cooperation, which remains limited and heavily reliant on U.S. facilitation rather than a fully integrated network. If Tokyo and Seoul cannot demonstrate track custody over one of the most heavily surveilled chokepoints in Northeast Asia, Beijing may calculate that stealth aircraft could more easily threaten key nodes in the opening hours of conflict. That perception alone erodes deterrence by suggesting that China could blind allied sensors at will.
Simultaneously, North Korea would be watching closely. Pyongyang has already invested in drones, cruise missiles, and electronic warfare capabilities designed to confuse or saturate South Korean defenses. North Korea is likely to draw lessons from the recent Ukrainian experience and adapt new tactics, though it remains uncertain how these adjustments would affect the existing U.S.–ROK defense posture. If the DPRK concludes that China’s stealth aircraft can slip through allied radars, it may be emboldened to launch its own operations under the cover of reduced situational awareness. In a two-front contingency, this could mean North Korean artillery and missile salvos coinciding with Chinese stealth strikes against command-and-control assets, overwhelming the allies’ ability to prioritize threats.
Finally, an undetected J-20 transit matters for the operational geometry of U.S. reinforcements. The Korea Strait is one of the few maritime corridors connecting U.S. and allied naval forces moving from the East China Sea into the Sea of Japan. If Chinese stealth fighters can patrol this route without detection, they gain the ability to shadow, harass, or even target allied naval movements during the most vulnerable phase of reinforcement. The result is a compounded risk: U.S. forces arriving late to Korea while already facing degraded air support, and Japanese and South Korean forces left uncertain about whether their back door is secure. In the event of a full-scale North Korean invasion, Chinese stealth fighters could disrupt the movement of U.S. Forces Japan to the Korean Peninsula and threaten Maritime Prepositioning Ships (MPS).
Operations in this corridor are not unprecedented; China has previously flown J-11s and bombers and sailed surface groups through the strait. But the introduction of fifth-generation stealth fighters signals a qualitatively different threat. It suggests that Beijing is probing allied radar coverage, compressing decision timelines, and preparing options to challenge allied situational awareness at the very moment it would matter most. In a crisis where minutes can determine whether reinforcements arrive intact, the absence of early detection could prove strategically decisive.
Countering Stealth: What Tokyo and Seoul Must Do
No fighter is truly invisible. South Korea has invested heavily in anti-stealth radar development, including Hanwha Systems’ VHF radars and an indigenous long-range air-defense radar, recently completed and widely reported to operate in the L-band. The Agency for Defense Development has also advanced passive radar concepts that exploit civilian broadcast signals. Combined with four E-737 Peace Eye airborne early warning aircraft—and prospective E-7 acquisitions—Seoul has the makings of a counter-stealth tripwire system.
Japan brings its own dense radar network, Aegis destroyers, E-2D AEW, and new SPY-7–equipped Aegis System Equipped Vessels. Yet in a Taiwan scenario, Japan would likely divert coverage east and south, leaving gaps in the north. That is where a joint approach becomes critical.
Concrete Japan–ROK joint measures should focus on five specific steps. First, the two countries should establish a joint counter-stealth radar network across the Korea Strait by jointly deploying relocatable VHF and L-band radars along with passive sensors from Jeju and Ulleungdo to western Kyushu, linking them into a single network to create a layered and redundant detection grid that neither nation could sustain on its own. Second, they must move from information sharing to real-time joint track custody; General Security of Military Information Agreement (GSOMIA) opened the political door, but the operational need is a combined track-fusion cell with agreed standards for sensor confidence, automated resolution of conflicting data, and direct feeds into both countries’ air-defense sectors. Third, Tokyo and Seoul should pre-design protected orbits for AWACS and tankers north of the strait, defending them with picket fighters and deception tactics so that J-20s cannot easily blind the allied air picture. Fourth, both governments should expand infrared search and track systems on fighters and coastal towers, since Infrared Search and Track (IRST) is immune to electronic attack and, when cued by passive radar, offers an additional means of tracking stealth aircraft in the strait’s challenging littoral environment. Finally, Japan and South Korea should conduct joint drills in which radars are deliberately silenced and only passive and airborne sensors maintain track custody, rehearsing how to turn a low-frequency radar hit into a weapons-quality track within minutes.
To achieve this, Washington, Tokyo, and Seoul must build on the 2023 Camp David Summit, as trilateral security cooperation is indispensable.
The Stakes
China’s broadcast never specified when the J-20 patrol occurred, and it is possible that allied sensors detected the transit without public disclosure. But Beijing showcased it before PLA Day and during heightened friction with Japan, signaling intent to normalize stealth patrols through a strategic chokepoint. This was less about tactical effect than about shaping perception: to suggest that even the most surveilled corridors can be penetrated.
If a Taiwan contingency coincides with a North Korean offensive, the outcome will hinge on whether the allies can see first, decide first, and shoot first. China’s J-20 flight through the Korea Strait was a rehearsal to make that nervous system blink. Japan and South Korea already have the tools to stop it. The question is whether they will connect them into a coherent counter-stealth belt before Beijing decides to test the system in combat. Given these dynamics, Tokyo and Seoul must strengthen their security cooperation despite lingering historical disputes.
[Photo by N509FZ, CC BY-SA 4.0, via Wikimedia Commons]
The views and opinions expressed in this article are those of the author.
Dr. Ju Hyung Kim currently serves as the president at the Security Management Institute, a defense think tank affiliated with the South Korean National Assembly. He has been involved in numerous defense projects and has provided consultation to several key organizations, including the Republic of Korea Joint Chiefs of Staff, the Defense Acquisition Program Administration, the Ministry of National Defense, the Korea Institute for Defense Analysis, the Agency for Defense Development, and the Korea Research Institute for Defense Technology Planning and Advancement. He holds a doctoral degree in international relations from the National Graduate Institute for Policy Studies (GRIPS) in Japan, a master’s degree in conflict management from the Johns Hopkins University School of Advanced International Studies (SAIS), and a degree in public policy from Seoul National University’s Graduate School of Public Administration (GSPA).
