Home / Articles / The Ryukyu Defense Line: Japan’s Response to China’s Naval Push into the Pacific Ocean
For most of the nineteenth and the first half of the twentieth centuries, the sea was the path over which foreign foes reached China, a fact not lost on Chinese naval strategists. In the 1980s, with the advent of long-range standoff weapons, those strategists began to argue that China would have to push out its seaboard defenses into the Pacific Ocean to ensure its security. They began referring to the waters between China’s coastline and what they called diyi daolian, or the “first island chain” (the Japanese home islands, the Ryukyu Islands, Taiwan, the Philippines, and Borneo), as an area where the Chinese navy needed to develop the capabilities to exert effective sea denial, if not sea control. Once that goal was achieved, China could push out its defensive depth to dier daolian, or the “second island chain” (the Marianas, Guam, and the Carolines).
Today, China routinely sends its warships beyond its “near seas.” But to do so, those warships must still pass through that first chain of islands, a key part of which are Japan’s Ryukyu Islands. Extending about 1,200 km southwest from Japan’s home islands towards Taiwan, the Ryukyu archipelago forms a natural boundary between the East China Sea and the rest of the Pacific Ocean. Its string of islands creates choke points through which much of the Chinese navy must transit to easily reach the “distant seas” of the Pacific Ocean. But Japan, nervous about China’s increasingly assertive behavior around the Senkaku Islands (Diaoyu in China), has begun to fortify the Ryukyu Islands with radar stations and anti-ship missile batteries. With enough of both sorts in place, Japan would be able to constrain and possibly bottle up a large part of the Chinese navy, if push ever came to shove between Beijing and Tokyo.
Long March to Distant Seas
But before China could exert any sort of sea control within the first or second island chains, it had to build a modern navy. And over the last three decades, it has done just that. By the 2010s, China had kicked its naval shipbuilding programs into high gear, producing no fewer than four nuclear-powered attack submarines, 14 diesel-electric attack submarines, 18 destroyers, 22 frigates, 50 corvettes, and 5 amphibious assault ships, collectively more vessels than are in the entire Royal Navy. And that does not even count its two new aircraft carriers and a third under construction. In the meantime, the Chinese navy began operating its warships in larger formations, further away from their home ports, and for longer periods than ever before.
Venturing into more “distant seas,” China has sent multi-ship flotillas as far north as the Arctic and as south as Australia and, on occasion, slipped nuclear-powered attack submarines into the Indian and Pacific Oceans. In 2012, it widely publicized the passage of a seven-ship task force through the Miyako Strait in the Ryukyu Islands. At the time, a former Chinese rear admiral, Yin Zhou, observed that such “distant seas training should be seen as ‘normal and routine’” and that “the West and Japan should get used to [it].” A year later, the Chinese navy held a major exercise called Maneuver-5 to practice a forced transit through the same waterway. Involving ships from all three of the navy’s fleets, the exercise was designed to not only improve their interoperability, but also hone their tactics under realistic conditions. Commenting on the exercise, a Chinese professor at Shanghai’s Fudan University said that it “tells Japan and the United States that they are not able to contain China within the first island chain.”
The Miyako Gap (and Others)
No doubt, China’s growing naval capabilities have concerned Japan. But even more worrisome to it have been the increasing frequency and forcefulness of Chinese incursions into its airspace and waters in the East China Sea, particularly around the Senkaku Islands, which Japan controls but China claims. That has led Japan to strengthen its defenses. One strategy uses the geography of the nearby Ryukyu Islands to its advantage. By establishing control over the choke points through the Ryukyu archipelago, Japan could limit the Chinese navy’s freedom of action. It could even restrict China’s North and East Sea Fleets from sailing into the “distant seas” of the Pacific Ocean (or attrit them as they attempted to do so) if a conflict were to break out.
Perhaps the most important of these choke points is the Miyako Strait, a 250 km-wide channel situated between the islands of Miyako and Okinawa. Already, Chinese warships routinely ply its waters. China’s navy clearly sees the strait as its main conduit between its coastal ports and the Pacific Ocean. Certainly, the Miyako Strait’s depth, the deepest of any strait in the Ryukyu Islands, would be valuable for the passage of submerged submarines. Indeed, a Chinese Shang-class submarine was detected doing so in 2018. Highlighting the Miyako Strait’s importance to China, its air force conducted large-scale exercises above the strait in 2016 and again in 2017. During one exercise, as many as 40 Chinese fighters and bombers flew over the strait, perhaps to rehearse how they could support a contested naval transit.
Clearly, Chinese military planners have already begun to consider ways to overcome a Japanese strategy to defend the Miyako Strait, though Tokyo is still in the process of implementing one. Throughout the Cold War and for years afterwards, Japan’s self-defense forces (i.e., its military) had been oriented northward to fend off the Soviet Union (and later Russia). Only in the last decade has Japan begun to shift its forces southwards, towards the East China Sea and the islands there. As a result, the Ryukyu Islands appear as though they are destined to occupy a place of high importance in Japanese defense strategy.
“A Ship’s a Fool to Fight a Fort”
For some forty years, Japan’s only meaningful military presence in the Ryukyu Islands had been an air base on Okinawa and three outlying early-warning sites. But with Chinese incursions into Japanese airspace and territorial waters rising seven-fold over the last decade, Tokyo has decided to beef up its capabilities in the area, starting with air and maritime domain awareness. In 2016, after years of preparation, Japan stationed a coastal observation unit on Yonaguni Island, at the westernmost end of the Ryukyu Islands and about 160 km from the Senkaku Islands. Equipped with mobile radar, high-frequency direction finding equipment, and two VHF/UHF monitoring systems, the unit can keep tabs on air and sea traffic not only near the two archipelagos, but also as far away as the Bashi Channel, between the Philippines and Taiwan. Together with Japan’s existing early-warning sites and growing constellation of surveillance satellites (its eighth went into orbit in February 2020), it has helped to give Japan a more complete picture of Chinese activity across the region. And, if rumors are to be believed, the unit might even serve as the western anchor of an underwater sonar array that has been strung along the Ryukyu archipelago to detect submerged submarines.
Meanwhile, Tokyo has started to establish a line of anti-ship missile batteries on the Ryukyu Islands. Given where the batteries will be situated, they will be able to cover all the choke points through the archipelago as well as parts of the East China Sea. In 2019, the first of those batteries, armed with Type 12 anti-ship missiles fired from mobile launchers, was activated on Amami Ōshima Island, the northernmost of the Ryukyu Islands. Then, in March 2020, Japan activated the 302nd Surface-to-Ship Missile Defense Battery, similarly armed, on Miyako Island. And finally, in the next year or so, a third anti-ship missile battery is expected to be activated on Ishigaki Island, between Miyako and Yonaguni Islands. Considering the possible threat from Chinese aircraft, Japan also plans to deploy an air defense unit, equipped with Type 03 medium-range surface-to-air missile (SAM) systems, alongside each of its anti-ship missile batteries.
Certainly, Japanese anti-ship missile units have risen to greater prominence. They have been featured in both the massive multinational RIMPAC 2018 and smaller bilateral Orient Sea 2019 exercises. By 2023, the units could become even more formidable when a new version of the Type 12 anti-ship missile is anticipated. It may have a range of 400 km, twice that of the current version, and a lower radar cross-section, making it stealthier. A line of batteries armed with such long-range, anti-ship missiles in the Ryukyu Islands could not only cover the Senkaku Islands, but also create a gauntlet of overlapping missile fires in the East China Sea. Battling their way through such land-based defenses would not be easy for warships. And that is what Japan is counting on. It apparently wants to prove the old naval adage true: “a ship’s a fool to fight a fort.”
Running the Gauntlet (or Around It)
For China’s North and East Sea Fleets to breakout into the Pacific Ocean, they would have to either overcome or avoid Japan’s anti-ship missile defenses in the Ryukyu Islands. The former strategy would likely begin with Chinese electronic jamming and suppression of Japanese air defense radars. Once the radars had been blinded, China would target the islands’ air defense and anti-ship missile batteries with ballistic- and cruise-missile strikes. Of course, Japan’s air base on Okinawa would be struck, too. Next, the Chinese air force could neutralize surviving anti-ship missile launchers and drive back any fighters dispatched from Japan’s home islands. If all went well, the Chinese navy could then sortie into the Pacific Ocean in relative safety.
But for all that to work, China would need to achieve a high degree of surprise. Failing to do so would give Japan the opportunity to disperse its mobile missile batteries, which could then use networked fire control to “shoot and scoot.” That would make China’s job of defeating them harder. To pick off individual launchers would require China to maintain continuous surveillance over the islands and, possibly, control of the airspace above them. While China could strike the island’s paved roadways to restrict the movement of Japanese launchers, their mobility would force China to use precious time and resources to hunt them down. Worse yet for China, with enough warning, Japan could seed the straits with naval mines, which would be equally difficult to detect and sweep before Chinese surface combatants, like aircraft carriers and destroyers, could confidently sail through the Ryukyu archipelago.
Chinese submarines face a somewhat different challenge. Even in peacetime, Japanese submarines probably patrol the western approaches to the Ryukyu Islands. From there, they would be well positioned to intercept their Chinese counterparts attempting to slip through the islands’ choke points, like the Miyako Strait. Of course, with a large enough submarine force (and a willingness to accept losses), China could try to force a transit through those choke points by overwhelming Japanese defenses. To get a head start, China could even preposition some of its submarines near the Ryukyu archipelago, so that they would be ready to dart through the straits as soon as hostilities were initiated. But if there was any warning, they would encounter not only more Japanese submarines, but also Japanese P-1 and P-3 anti-submarine warfare aircraft, perhaps even more so on the eastern side of the Ryukyu Islands, where the aircraft could operate with a bigger buffer between them and China’s land-based fighters.
A better strategy for China might be to avoid the Ryukyu defense line’s strong points and roll it up from the south with the Chinese navy’s South Sea Fleet. Steaming north through the Bashi Channel or Luzon Strait, a Chinese carrier-led task force would put Japan’s anti-ship missile defenses at a disadvantage. Rather than having to confront overlapping fires from multiple batteries, the task force would only have to face one battery at a time. Japanese commanders would then be forced to choose between launching a small missile salvo from a single battery (which would have a lower chance of penetrating Chinese shipboard missile defenses) or waiting for China’s task force to come into range of more batteries to launch a larger missile salvo. Unfortunately for those commanders, waiting would give China more time to locate and destroy their anti-ship missile batteries.
To prevent such a scenario from unfolding, Japan could attempt to defeat the approaching Chinese task force before it ever reached the Ryukyu Islands. The most likely place to do so would be in the waters southeast of Taiwan, where Chinese land-based fighters would be operating near the limits of their unrefueled combat range. That would help even the terms of engagement if Japan was to send its own carrier-led task force to counter China’s. Even better for Japan, its navy has begun to acquire Taigei-class diesel-electric attack submarines that are capable of cruising over long distances at high speeds. Japan could use them to attrit China’s task force as it sailed northward from the South China Sea, giving Japanese forces an even bigger edge.
On the other hand, China could use its Shang-class nuclear attack submarines, which are based with the South Sea Fleet, to range farther north to attack shipping near Japan’s home islands. Such attacks could draw Japanese forces back from their forward positions, leaving defenses around the Ryukyu Islands spread thin. China could then try to overpower Japan’s weakened defenses and gain access to the Pacific Ocean. Without question, Japan’s ability to contain Chinese naval forces within the “first island chain” is by no means assured. But neither is China’s success in penetrating it without substantial losses.
A Longer March?
In the meantime, new military technologies continue to emerge. Possibly the one with the biggest impact on the Ryukyu defense line is the advent of hypersonic missiles and glide vehicles which can travel at over five times faster than the speed of sound to hit their targets. Both China and Japan are developing them. Such missiles would surely improve China’s chances of achieving surprise. But in the hands of Japan, they would make a forced transit through the straits of the Ryukyu Islands that much more perilous. In fact, Tokyo already has plans to arm two anti-ship missile battalions that will be responsible for the defense of those islands with hypersonic munitions.
China might be making a bid for “maritime primacy” in the Indo-Pacific region, but it must first get its navy beyond the Ryukyu Islands. That entails not only sailing its naval forces into the Pacific Ocean, but also keeping them safe and supplied once they are there. Japan’s growing defenses in the Ryukyu Islands complicate China’s ability to do either. China may have built a powerful navy, even a blue water one, but no one said that getting it into the blue waters of the Pacific Ocean would be simple.
The views expressed in this article are those of the author alone and do not necessarily reflect the position of the Foreign Policy Research Institute, a non-partisan organization that seeks to publish well-argued, policy-oriented articles on American foreign policy and national security priorities.
 China refers to its “near seas” as the waters of the Yellow Sea, East China Sea, and South China Sea and its “distant seas” as those beyond them.
 “Rear Admiral: ‘Maneuver-5’ Exercise Is the First High-Sea Confrontation Training in Real Sense,” Jiefangjun Bao, Nov. 4, 2013; and “PLA Navy ‘Maneuver-5’ Exercise Enters Phase of Drills with Real Troops,” CCTV, Oct. 25, 2013.
 Gabriel Dominguez, “Japan identifies vessel spotted near disputed islands as a PLAN Shang-class submarine,” Jane’s Defence Weekly, Jan. 16, 2018.
 Gabriel Dominguez, “PLAAF sends fighters, bombers on ‘regular patrol exercise’ around Taiwan,” Jane’s Defence Weekly, Dec. 13, 2017; Gabriel Dominguez, “Japan JASDF scrambles fighters as Chinese H-6 bombers fly near Okinawa,” Jane’s Defence Weekly, Jul. 14, 2017; Jesse Johnston, “ASDF scrambles jets as China sends more fighters, bombers through Miyako Strait as part of large drill,” Japan Times, Mar. 3, 2017; and “China’s air force says recent long-range drills routine,” Reuters, Dec. 15, 2016.
 Desmond Ball and Richard Tanter, The Tools of Owatatsumi: Japan’s Ocean Surveillance and Coastal Defence Capabilities (Canberra: Australian National University Press, 2015), pp. 22-27.
 Since Japan’s coastal observation unit on Yonaguni Island is reportedly “modeled” on Japan’s 301st Coast Observation Unit and the latter unit operates a LQO-3 underwater sonar array, the unit on Yonaguni Island has been speculated to operate one too, particularly in light of a rumor that the United States, in cooperation with Japan, had laid an underwater sonar array along the Ryukyu Islands in the late 2000s. Ball and Tanter, The Tools of Owatatsumi, pp. 58-62; and “美日聯手在中國潛艇基地周圍設置水下監聽網_大陸頻道” [“U.S. and Japan Work Together to Establish Undersea Listening Network Close to Chinese Submarine Bases”], Beijing Daily, July 10, 2013, dailynews.sina.com/bg/chn/chnmilitary/sinacn/20130710/00164728365.html.
 Japan Ministry of Defense, Defense of Japan 2020, Mar. 31, 2020, p. 252-253; and Kosuke Takahashi, “Japan sets up three military camps on southwestern islands,” Jane’s Defence Weekly, Mar. 26, 2019.
 Japan Ministry of Defense, National Defense Program Guidelines for FY 2019 and Beyond, Dec. 18, 2018, p. 33.