By your own admission a defensive security guarantee will "kick off a great power war..." which is a way of saying Russia can't be trusted to keep the peace, no? Not that I disagree but I'm not sure it's as strong an argument against Ukraine's request as you think it is. It may have even been part of Neville Chamberlain's notes on the subject.
the chamberlain/weimar comparison is inaccurate for this case. germany was an ascendant power; russia is a crippled one. the war was actually a great investment because it's decimated russia's military population and stores of materiel and cut her off from the world enough to severely damage her economy. it will take substantial time for her to rebuild.
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?
