I just checked: only Moldova is left that's not in NATO and in place where Russians could march to (but they would need to get the western Ukraine first). Poland, Slovakia, Lithuania, Estonia, Latvia are all under NATO nukes protection; Sweden and Finland will join shortly. Are you sure Putin - and all the people around, at the same time! - are insane enough to risk eradication of all its cities?

Nuclear weapons changed the circumstances so much that direct comparison with 1938-1939 is impossible. We neither can give Slovakia away, nor have we a reason to. On the other hand, for Ukraine, swallowing the bitter pill and losing territory would also clear the way for NATO application (the civil war in the east makes it de iure impossible) and would give them time to prepare to defend from much better fortified positions, with much greater access to Western weaponry, and with much better contingency plans.