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participated in the last two iterations, and was scheduled to join this summer's exercise until its invitation was withdrawn.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.