I'm not so sure regarding Taiwan, as many people are in an isolationist mood and we don't have a formal mutual defense pact with Taiwan.
South Korea is a 50/50 chance, as we do have a formal mutual defense pact with that country but again many people are in an isolationist mood.
Japan is a 75/25 chance, as not only do we have a formal mutual defense pact with that country, but it also shares a direct maritime border with us. Should China invade Japan (or Russia, for that matter), we would then have Chinese troops right on our doorstep.