The U.S. doesn’t have any claims in the region, so a bit much to say they’ve been risking war with us. I don’t see how we can make a case for being more belligerent in a territory dispute than the sides involved.
We’ve drawn our lines pretty clearly: Japan is a key U.S. ally, so their islands are off limits. The other countries, not so much. I suspect if U.S.-Philippine relations were doing better and the Philippines were willing to place their own forces at risk, we’d back them up, but that doesn’t look like its going to happen.