The U.S. has long held that the U.S.–Philippines Mutual Defense Treaty does not extend to Philippine claims in the South China Sea. There may have been a time when that position was prudent. After all, what interest could the U.S. have had getting involved in a spat among six relatively weak claimants?

That time has past. Chinese “fishing patrol boats” and its rapid military modernization have chased it away.

There is now one claimant stronger than the others, growing stronger by the day, and feeling it. We need to introduce some uncertainty into Chinese calculations.

Let’s be clear about one thing: Tensions in the South China Sea are about China. Chinese diplomats, “friends of China” and the faint-hearted will cloud the issue. They will claim that the U.S. is taking sides in sovereignty disputes that shouldn’t concern it and that its action is a problem for all the parties to the conflict.

This response is going to call for some common-sense treatment. Is anyone concerned with Bruneian or Malaysian aggression in the South China Sea? Is it Vietnamese patrol boats challenging Philippine oil exploration off its coasts? The Taiwanese have all they can handle in the Taiwan Strait. Their recent activity is aimed at simply giving them the voice they deserve in conversations from which they are systematically excluded.

Strategic ambiguity is not that difficult to pull off. Imagine an exchange along the following lines:

Reporter: “There are voices calling for the U.S. to apply the U.S.–Philippines Mutual Defense Treaty to the territorial dispute in the South China Sea between the China and the Philippines. Does the treaty apply to these territories?”

State Department Rep: “We are absolutely committed to our treaty with the Philippines. There very well may be circumstances under which the treaty would apply to Chinese activity in Philippine-claimed territory.”

Reporter: “What would those circumstances be?”

State Department Rep: “I don’t want to speculate, but keep in mind the expansiveness and intrusiveness of the Chinese claim. It actually includes islands currently occupied and administered by the Philippines. As a treaty ally of the Philippines, we are greatly concerned by provocative action taken by the Chinese anywhere within the disputed area.”

Secretary of State Hillary Clinton’s diplomatic initiative in the South China Sea was a welcome one. But it’s time to start making the Chinese think about consequences beyond diplomatic ones. Strategic ambiguity has helped maintain peace and stability in the Taiwan Strait. The same concept could help protect the Philippines and peace and stability in the South China Sea.