Over the last several years Sino-Japanese relations have reached low after new low—all thanks to claims and counterclaims over the Senkaku islands (China refers to them as the Diaoyu islands). The relationship between the two countries, which had been tepid at best—quickly cooled beginning in 2010 as both sides jockeyed for position over the disputed islands.

Japan’s Cold War adversary, the Soviet Union, threatened the country primarily in the north with submarines, bombers, fighters, and a theoretical invasion by sea. China is a different sort of strategic threat to Japan, being most active in the more southern East China Sea, with its military reach extending to the Senkaku and Ryukyu Islands as well as the Japanese mainland.

The challenge posed by the People’s Liberation Army has shaken a complacent Japanese government, which had left its national security establishment virtually unchanged since the 1980s. A national security council similar to that in the United States has been formed, secrecy laws have been passed and Japan’s defenses are shifting southward. Here are a five weapon systems that Tokyo should worry about as tensions with Beijing continue to simmer:

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J-20 Stealth Fighter:

Japan lost control of her airspace during the Second World War to devastating effect. The result: as many as 900,000 people killed in aerial bombing raids. Since then, Japan has invested in only the best American fighters.

Unfortunately, American refusal to sell the F-22 Raptor overseas and the aging of the F-15J fighter is set to create a window of vulnerability. Even worse news for Japan, China is developing something that could push through Tokyo’s airspace: the J-20 fighter.

China’s first 5th generation fighter, the J-20 is a large, delta-winged aircraft with a long, broad fuselage ending in two turbofan engines. The aircraft is in advanced stages of development. There are six demonstrators flying, with the last two spotted in November and December of last year. The U.S. Air Force estimates the aircraft will become operational some time between 2017 and 2019.

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The J-20 is a long-range combat aircraft with a stealthy profile. The aircraft’s sensor suite will probably include a AESA phased array radar and an electro-optical targeting system. These will provide targeting for weapons carried in the plane’s two large internal weapons bays. Conceivable payloads include air-to-air, land attack or anti-ship missiles.

Nobody really knows what role the J-20 will be assigned. The aircraft’s large frame points to an air superiority or multi-role fighter with a large payload and long range. Like the F-15 Eagle, it could become both. The most obvious role for the J-20 is as a fighter to directly challenge the F-15J. The J-20’s stealthy capabilities and advanced avionics could give Chinese pilots a decisive edge.

China could also use the J-20 to intercept Japanese support aircraft. Japan has few bases in the area capable of supporting the Senkakus. In the event Naha Air Base on Okinawa were knocked out, Japanese fighters would have to fly from the southern island of Kyushu. In either case, Japanese tankers would be critical toward keeping.

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S-400 Surface to Air Missile System:

According to reports, China seems set to purchase (or is very close to purchasing—reports on this vary) six battalions of S-400s. Each S-400 battalion consists of a command post, radar systems and twelve launch vehicles, each with four 40N6 missiles.

The S-400 is a highly sophisticated system and would be a boon to Beijing's air-defense capabilities. The radar system can track up to one hundred targets at a time and engage twelve simultaneously. It also has some limited capability to detect stealth aircraft.

The 40N6 missile has a range of 400 kilometers. Deployed on China’s coastline, it brings all of Taiwan’s airspace into range. Similarly and of particular concern to the Japanese, it will also be able to put the Senkaku Islands under range.

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Although the Senkakus will be at the outer edge of the S-400’s engagement envelope, this is still a serious problem for Japan. Japan regularly patrols the Senkaku Islands by air, with both P-3C Orion maritime patrol planes and F-15J fighters based at Okinawa.

A very possible purchase and later deployment of S-400s near the Senkaku Islands would create difficulties for Japan in both peace and war. In peacetime, it could mean scaling back patrols over the tiny islands by slower, less maneuverable aircraft, such as the P-3C. In the event of an incident, such aircraft could be caught within range of the S-400 and would have little chance of survival.

Likewise, in wartime the S-400 could help China maintain air superiority over the islands. China has a limited number of aerial refueling tankers, and in a conflict might not be able to provide round-the-clock combat air patrols over the islands.

Japan has not previously had to deal with land-based air defense systems as a threat to its aircraft. The future may see greater Japanese investment in electronic warfare aircraft capable of jamming the S-400 and systems like it.

Type 071 Landing Platform Dock:

In the event of conflict over the Senkaku Islands, the most visible way for China to demonstrate some sort of victory would be to put ground troops on the islands. Landing troops would force Japan to acquiesce or eject them by force. An amphibious landing would be the only way to deliver troops to the islands.

Fortunately, the People’s Liberation Army Navy has just the ships to send. The Type 071 landing platform dock ships are capable of landing Chinese marines on remote beachheads by air and sea. There are four operational Type 071 ships, with another one currently fitting out and a sixth one planned. All are named after mountain ranges.

Each ship displaces 20,000 tons and is nearly 700 feet long. Ships of the class can carry a battalion of marines—roughly 400 to 800 troops—and around 18 armored vehicles. They are roughly analogous to the U.S. Navy’s San Antonio-class LPDs and the Royal Navy’s HMS Ocean.

The ship is equipped with both a well deck for launching amphibious craft and a helicopter hangar and landing deck for launching helicopters. The well deck can carry up to four Chinese copies of the American LCAC hovercraft, each of which can carry one tank or two or more lighter vehicles. The 071 ships will typically carry 2-4 Z-8 heavy transport helicopters, each of which can carry up to 24 soldiers.

In a Senkakus invasion scenario, a single 071’s helicopters could land a company’s worth of infantry to secure one of the islands. Once secure, hovercraft could bring in supplies, heavy weapons, air defenses and even anti-ship missiles to keep Japanese forces at bay.

The first three of the ships serve with the South Sea Fleet facing Taiwan and the South China Sea, but the fourth serves with the East Sea Fleet, and the fifth and sixth are also projected to serve with the East Sea Fleet.

DF-21A Medium Range Ballistic Missile:

China has a vast arsenal of conventionally armed short and medium range ballistic missiles. Originally designed to threaten Taiwan, China’s conventional ballistic missiles have in time grown in accuracy and range, to the point where they could now be used to strike targets across Japan.

The DF-21A medium range ballistic missile is a solid fueled ballistic missile of great precision. Operational since the mid-1990s, the DF-21A has a range of up to 2,150 kilometers, placing all of Japan within striking range from the Chinese mainland.

The DF-21A carries a single 500-kilogram warhead, which can be a conventional unitary warhead, conventional submunition, chemical or even nuclear. (That having been said, China has a “no first use” policy on nuclear weapons, which would likely preclude the use of Chinese nukes against a non-nuclear Japanese state.)

A conventionally armed DF-21 would be very useful in a conflict against Japan, and could be used against a variety of targets. Airfields, energy storage facilities, government facilities, radar and air defense facilities, army bases, air force bases, and naval bases could all be struck with some precision.

An unknown number of DF-21s have been fielded. It should also be noted that the DF-21A is also part of China’s regional nuclear deterrent, so even in the event of war many launchers and missiles would be held back from conventional missions to maintain a medium range nuclear deterrent.

A variant of the DF-21, the DF-21D, is the basis for China’s anti-ship ballistic missile. The ASBM, as it is known, is a cornerstone in China’s anti-access, area-denial strategy (A2/AD) for keeping the U.S. Navy out of the Western Pacific.

Il-78 tanker:

If China and Japan were to come to blows, China would need to beat Japan in the air. The Senkaku/Diaoyu Islands are 400 kilometers from the Chinese mainland. Most of the People’s Liberation Army Air Force’s modern fighters, including the J-10, J-11, and Su-30MKK, can reach the islands.

The rest of Japan — including Okinawa, Kyushu, and even Honshu — is at least 700 kilometers from the Chinese mainland. Smaller, shorter-legged planes such as the J-10 can’t get their on their own and will need the ability to refuel in midair. The ability to refuel in midair also increases the number of aircraft and airfields in China that can participate in a conflict.

The Il-78 tanker is a critical link in the People’s Liberation Army Air Force’s power projection strategy. The Il-78 is a Russian Il-76 cargo aircraft converted to aerial tanker, and carries enough fuel to fill the tanks of a J-10 multirole fighter 20 times over. Trailing three refueling hoses behind it, the Il-78 can refuel three aircraft at once.

China tried to purchase a number of Il-78 tankers from Russia in 2005, but the Uzbekistan plant that Russia relied on for manufacturing the airframe was unable to produce them. However, China has reportedly purchased three from Ukraine, and at least one of them appears to have been delivered, based on satellite imagery from late last year.  Beijing has also signed deals for more Il-76 planes, likely to convert them to tankers, but production problems in Russia have mean only a handful have been delivered. Until China addresses its tanker shortfall, the PLAAF’s ability to project substantial airpower in a conflict with Japan will be seriously limited.

In wartime such a critical capability will be a critical vulnerability. If China’s tankers were to be shot down, the PLAAF’s reach would be shortened dramatically, limiting its ability to prosecute the war. This will make the tankers prime targets for the Japanese Air Self Defense Force.

Kyle Mizokami is a writer based in San Francisco who has appeared in The Diplomat, Foreign Policy, War is Boring and The Daily Beast. In 2009 he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami.

Image: Wikimedia Commons