Do you feel that the Russia/Ukraine war is a proxy war? If so, how do you explain that Russia invaded Ukraine? Generally, a proxy war is considered to be one in which the instigating party does not actually take part, at least, not technically. So, are you saying that the US/NATO instigated the war?
As for a hypothetical war between China dn Taiwan, that is super tricky, because everyone but China believes that Taiwan is a sovereign nation, but pretends like that don't think that to keep from China getting their panties in a twist. So, would the US be willing to pretend it is a civil war, (What you're trying to be cute and calling a "war with itself") or would they say it was an act of aggression between a larger nation and a smaller one? Damned if I know. I would honestly say that's above my pay grade, except that I'm not even in that pay scale.
The bigger question, of course, is "Can there be a just war?" My view is yes, but they're pretty few and far between in our modern world. The most common, perhaps only case for it, is when a nation is purely protecting itself. Ukraine, Jordan, going back to WWII. Those are the only ones I can think of in living memory that I would consider a just war. So what about the Civil War? Was the US just in defending itself from a breakaway group? Or the revolutionary war, was the soon-to-be US just to break away itself?