We already have. The US has pretty consistently stated that it sees the Spratlys, Scarborough Shoal, etc as disputed territory, not part of the Philippines. Of course we also say that the disputes should be settled peacefully, but realistically I doubt the US response to a seizure of Kalayaan Island would go much beyond verbiage and a few arms sales to Manila. I don't see the US wanting to go to a military confrontation over the islands. Some kind of economic reprisal, possibly. I'm not sure what the options are on that side, would be interesting to see discussion from people who know more about it.
I actually don't expect this to happen yet, though it could, especially if the Chinese government feels a pressing need to rally some nationalist spirit by beating up someone who can't fight back. More likely we'll see some fishing boats seized and/or chased out, more harassment of oil exploration missions, possibly an ADIZ over the South China Sea. These threats have been tossed around before, usually through semi-official channels.
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