TOKYO -- China is pushing ahead with its artificial island building in the South China Sea, where it plans airstrips and other facilities.

Reports in Japan say the U.S. military is feeling a growing sense of urgency over China's aggresive island-making projects. But Japan should be feeling even more anxiety.

Many analysts say China is moving to secure seabed resources from around its artificial islets. But it could easily go further and even one day drive a military wedge between Japan and the U.S.

Lying at the very heart of the issue is the balance of nuclear power between the U.S. and China.

The decoupling goal

China lacks an effective submarine-launched ballistic missile system that would allow it to retaliate against a surprise nuclear attack.

A Chinese Xia-class nuclear-powered submarine (Kyodo photo)

It once developed and built a Xia-class nuclear-powered submarine meant to carry ballistic missiles, but the submersible turned out to be little more than a paper tiger.

With no nuclear deterrent to match the U.S., China is building new Jin-class nuclear-powered submarines. It apparently wants three such vessels, which it would arm with nuclear weapons, by 2020 and deploy one of them in the South China Sea.

China would also need to deploy troops to protect a Jin-class sub in the South China Sea.

In addition, the country is looking to someday deploy multiple aircraft carriers, apparently to boost its defense capabilities in the South China Sea and prevent U.S. fighter jets and patrol aircraft from getting close to the region.

In the event of a military contingency, however, an aircraft carrier could be attacked and sunk by enemy forces. So China could be thinking of the airstrips it is laying down on its man-made islets as unsinkable aircraft carriers.

China's submarine-launched ballistic missile system will not pose an immediate threat to the U.S. because the number of missiles China could deploy remains limited, as does their range.

But the situation will change if China's aggressive building in the South China Sea continues unchecked.

China will make progress in turning the South China Sea into its own military sanctuary; it is already denying access to the region by, among other tactics, deploying Russian-made high-performance air-defense missiles and other weapons on islands.

If it gains the ability to fire more missiles from submarines in the South China Sea -- and obtain longer-range missile technology -- it could aim those projectiles at the U.S. mainland.

The U.S. might then hesitate to defend Japan, which could suddenly find itself out from under the U.S.'s nuclear umbrella.

With the U.S. and Japan thus militarily decoupled, China would then be able to threaten Japan or encroach on Japanese territory, with the U.S. playing the role of paper tiger.

This is why Japan should be feeling anxious.

Submarine hunter

The U.S. military has recently started stepping up warning and surveillance activities in the region in partnership with Japan and Southeast Asian countries that have long-running territorial disputes with China in the South China Sea.

Japan's Maritime Self-Defense Force is one of the world's best submarine hunters. Even the U.S. military acknowledges the force's superior antisubmarine patrolling capabilities.

The U.S. will likely continue to face tough restrictions on defense spending in the foreseeable future. It would be quite natural for the U.S. to ask Japan's Self-Defense Forces to step up their participation in the warning and surveillance activities.

Japan, which has ruled out nuclear arms for itself, has no choice but to join the U.S. in keeping an eye on what China is up to in the South China Sea -- and trying to prevent the country from making the body of water its own military sanctuary.

The Self-Defense Forces will probably need to set up a second overseas operating base, this time somewhere in the South China Sea; its first is in Djibouti, a small African country along the Gulf of Aden, that allows Japanese forces to engage in anti-piracy operations in waters off Somalia.

The base could land in Singapore or the Philippines. Given that Manila, like Tokyo and Washington, is deeply concerned about China's military buildup in the area, the Philippines is more likely.