you can make a "what about czechoslovakia/poland/nazis" argument about heavy intervention in what would otherwise be any proxy war. you say czechoslovakia, i say vietnam, i say korea, i say the middle east.
the American interest in this war isn't so much "we love the ukraine" as "this is an effective way to cripple russia for the next decade by proxy". by doing so, we avoid that situation in a much smarter way than chamberlain. and because russia wasn't in that great of a spot to start with, a protracted war of attrition is really bad for her.
are you suggesting we should begin a war against russia, historically a massively losing proposition, over a couple oblasts of the ukraine? again, how many americans should we send off to die? how much should we weaken our resources for a much more concerning conflict with china?
the whole "sending ukraine materiel is going to cause WWIII" thing is sort of bs russian propaganda. the idea that direct American military intervention isn't risking that is very much not.
again, how many Americans do you think it's appropriate for their own government to sign up to die for this cause? i think the number is zero. if you think it's higher, i'd very much like to understand why and how many you think is a reasonable number. i understand it'd be a ballpark figure, not a bright line, but i'd like at least an order of magnitude grasp of what people think is appropriate and why they think so.