> After all, NATO and the USSR (along the Warsaw Pact) used to be sworn enemies for decades by then.
USSR was literally Russia + occupied countries. Saying it is understandable is like saying it would be understandable for Germany to attack Poland and France again, because Third Reich went that far too.
Part of the reason why the German (traditionally Prussian) military elite supported the Nazis was exactly that, the loss of previously Prussian (or German) territory. And not territory that was invaded but couldn't be held. Again, I am not defending Putin or Russia here.
USSR was literally Russia + occupied countries. Saying it is understandable is like saying it would be understandable for Germany to attack Poland and France again, because Third Reich went that far too.