They won't. What you expect, US going to war with a nuclear power over some territory in some third-rate country? The maximum you can expect is a couple of Obama speeches and Kerry promising an unbelievably small diplomatic effort to solve it.
So yes, all those treaties are worth something only when everybody wants to follow them, and they stop being worth paper on which they are printed as soon as one of the participants decides to break them. That's how it always worked.
It's not just "some territory": Russian natural gas --- one of the principle sources of Russian influence in Europe --- goes through Ukrainian pipelines.
So yes, all those treaties are worth something only when everybody wants to follow them, and they stop being worth paper on which they are printed as soon as one of the participants decides to break them. That's how it always worked.