SINGAPORE — While addressing the Shangri-La Dialogue defense summit in Southeast Asia, Philippine President Ferdinand Marcos Jr. set near black-and-white criteria on what would count as an act of war by China.

The comment came in reference to a question about Second Thomas Shoal, a reef in the South China Sea where the Philippines have an outpost. Despite a 2016 UN ruling to the contrary, Beijing claims ownership over the territory and has harassed Filipino vessels resupplying the base each month. That harassment has included the firing of water cannons and ramming of a Filipino vessel.

But were that aggression to result in the death of a Filipino citizen, Marcos said, it would be “very, very close to what we define as an act of war.”

That’s more than semantics.

The Philippines and America have a mutual defense treaty, dating back to the 1950s. Partly for that reason, China’s behavior around the reef has been in the “gray zone,” or activity that falls short of full conflict. Beijing uses its Coast Guard rather than its Navy to intercept Filipino vessels. And while those forces have injured Filipinos, they haven’t killed anyone.

The question then is what Manila defines as an act of war instead of harassment — and hence what could spark a war between the U.S. and China.

“Almost certainly, it’s going to be a red line,” Marcos said of a situation in which Chinese forces kill a Filipino citizen.

Asked after his speech later in the weekend whether the U.S. would also consider that an act of war, U.S. Secretary of Defense Lloyd Austin said he wouldn’t speculate on hypothetical situations.

“I will continue to emphasize that our commitment to the mutual defense treaty is ironclad,” Austin said.

The keynote address was a high platform for Marcos, and the Philippines more broadly. Manila is playing a larger role in the region’s security as it deepens its security ties with Washington and other like-minded countries, such as Japan and Australia.

There’s no more unifying issue for that coalition than the South China Sea, said Greg Poling, an expert on Southeast Asia at the Center for International and Strategic Studies.

“The South China Sea, in one way or another, resonates with pretty much all U.S. allies and partners — even those outside the region — because it’s a question overall law,” Poling said. “Everyone agrees that China’s in the wrong.”

This story has been updated with comments from Secretary of Defense Lloyd Austin.

Noah Robertson is the Pentagon reporter at Defense News. He previously covered national security for the Christian Science Monitor. He holds a bachelor’s degree in English and government from the College of William & Mary in his hometown of Williamsburg, Virginia.

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