Today American and Chinese warships are leaving Guam … together. Not as enemies, but as friends. Having sunk several floating targets in a gunnery exercise, the combined task force is continuing on to Pearl Harbor. There the U.S. and Chinese vessels will join the world’s largest naval war multinational exercise, altogether involving 22 countries.

America’s biannual Rim of the Pacific war game, or RIMPAC, kicks off on June 26. This year Australia, Brunei, Canada, Chile, Colombia, France, India, Indonesia, Japan, Malaysia, Mexico, The Netherlands, New Zealand, Norway, Peru, South Korea, The Philippines, Singapore, Thailand, Tonga and the United Kingdom are all participating.

It’s the first time China has sent ships. And not everyone is thrilled.

RIMPAC dates back to 1971. The United States needed to maintain credibility as a military power in Asia as it withdrew troops from Vietnam. A huge naval war game was just the thing. At RIMPAC, ships, planes, subs and other naval forces simulate intensive naval battles. Officers and crews swap tactics. The idea is for every navy to sail away from the war game a better fighting force—and a better ally.

For decades, China understandably sat out RIMPAC. Especially lately, China has violently clashed with its neighbors over disputed islands in the China Seas.

Beijing isn’t a great ally ... to anyone, really. Indeed, the U.S. and many other navies tailor their ships and tactics specifically for defeating Chinese forces, in the unlikely event of major war.