A proxy war? Who is using Russia as a proxy? Words have meanings, you know. This is a war of conquest, and a very direct one at that. You can tell by how the aggressor has already legally (as in “Russian law”, not “international law”) incorporated parts of the defendant’s territory into itself.
Also there’s exactly two reasons why the US is in this: a) glee at Russia willingly running into another Afghanistan and b) because Europe is. The US can’t countenance the impression that Europe does military things without it but if Trump were to be elected tomorrow and turned the country to isolationism European support for Ukraine would stand fast.