We haven’t really tested the idea of a geographically restricted war. During the Cold War there were some pretty transparent proxy wars, but the proxy still allowed for backing out and saving face.

I don’t think geographically restricting a war is even possible, really. The US’s typical game plan involves hitting the enemy’s decision-making capabilities faster than they can react. That goes out the window if we can’t hit each other’s mainlands. A war where we don’t get to use our strongest trick and China keeps their massive industrial base is an absurd losing one that the US would be totally nuts to sign up for.

Anyway, we and China can be perfectly good peaceful competitors.