The U.S. Department of Defense has decided that China is no longer welcome at its pre-eminent Indo-Pacific multilateral naval exercise. The chief cause of Pentagon pique is Beijing's island building in the South China Sea and the ensuing militarization of them despite a pledge not to do so. The U.S. move will not stop the creeping expansion of the Chinese defense perimeter but it is a signal that such behavior will have consequences. If the message is to have any impact on China, the United States cannot be the only country that sends it.

The U.S. Pacific Command, headquartered in Hawaii, holds the Rim of the Pacific (RIMPAC) military exercise every other year. First held in 1971, it has slowly expanded. In the last iteration (in 2016) 26 countries, including Japan, provided more than 40 ships and submarines, more than 200 aircraft and 25,000 personnel for the month-long program. China participated in the last two iterations, and was scheduled to join this summer's exercise until its invitation was withdrawn.

A U.S. Navy spokesperson justified the revocation by pointing to the deployment of anti-ship and surface-to-air missile systems on three of the islands that it built in the Spratly Island chain west of the Philippines. Those are only the most recent steps in an extensive militarization program. China has built deep-water ports, aircraft hangars, communication and other administration facilities.

On Subi island, there are now 400 buildings, and experts expect it to become an outpost for hundreds of marines. Some of the islands now have runways long enough to accommodate long-range aircraft on the islands; in recent weeks. China landed nuclear-capable bombers on other disputed territory that it has developed. Beijing is also reported to have installed military jamming equipment on the islands in recent months.

U.S. Adm. Philip Davidson, nominated to take charge of Pacific Command, told Congress that China's People's Liberation Army "will be able to use these bases to challenge U.S. presence in the region, and any forces deployed to the islands would easily overwhelm the military forces of any other South China Sea claimants." With construction of "forward operating bases" now complete, "the only thing lacking are the deployed forces," said Davidson.

That development is alarming, but U.S. officials seem particularly offended because China had promised not to do so. When he met then-U.S. President Barack Obama in 2015, Chinese President Xi Jinping said that "China does not intend to pursue militarization."

Chinese Foreign Minister Wang Yi called the decision to revoke the invitation "unconstructive," arguing that his country is only building "necessary defense facilities on our own islands," which is "the right to self-defense and preservation of every sovereign state." A foreign ministry spokesperson added that the building program "has nothing to do with militarization," an assertion that China would dismiss if offered to it.

In the 1980s, Prime Minister Yasuhiro Nakasone reportedly claimed that Japan would be an "unsinkable aircraft carrier" against the Soviet Union, and that logic drives Chinese behavior today. China is expected to replicate the current deployments further south and step up the tempo of operations to make the Chinese presence real and substantial. Then, as Adm. Davidson explained, "China will be able to extend its influence thousands of miles to the south and project power deep into Oceania."

The U.S. decision to revoke the RIMPAC invitation is a slap on the wrist, but nothing more. Other countries must join Washington in making clear their concern about Beijing's behavior. Japan should be prominent among them. The extension of China's defense perimeter can threaten Japan's Senkaku Islands and makes possible interference with the sea lines of communication that support and sustain the Japanese economy.

Tokyo should therefore be leading regional diplomacy. Japan, with like-minded countries, should be showing the world the full extent of Chinese militarization of its South China Sea outposts. China will object to a public airing of its actions but offended governments must not be deterred. The problem is China's island building and fortification, not the recognition of such behavior. Japan should demand Chinese adherence to international law — a position that it is better positioned to defend than even the U.S. given Washington's failure to ratify the Law of the Sea treaty — and continually assert the importance of rule of law to regional order.

Japan, like the U.S. and other concerned governments, must not go out of its way to provoke China but it should not ignore its attempts to rewrite the status quo. Bad behavior must not be rewarded. This is no time for business as usual.