Marcos has been showing a LOT of restraint. I think he fears a larger conflict with China, even though we would be obligated to side with Manila by treaty.

Were I him, I would say that I will be enforcing our internationally recognized territorial waters, up to and including firing on vessels that do not have permission to be in said waters. If anyone is silly enough to fire back, we (Manila) will invoke the defense treaty with the USA. But I'm not Marcos. Hopefully he'll do something like that soon. Xi is not going to stop until someone stops him. Xi has to be forced to do some math, and figure out the costs of continuing will be far greater than the benefits. Right now, it's costing him nothing.

It's history rhyming again. About 90 years ago a nation started being aggressive in territorial disputes. They ended up occupying the Rhineland, the Ruhr, the Sudetenland (and then the rest of Czechoslovakia), and Austria...all without fire a shot, and all without much more than the nervous wringing of hands by nations that could have stopped it. That autocratic leader didn't stop, and neither will Xi (or Putin, for that matter). Not without some significant push-back.