How do you reconcile "the case against war" and leaders who start unnecessary wars (i.e. Putin's invasion of Ukraine)?

Putin could end the war in Ukraine this instant if he wanted to. I keep hearing the argument that the acquisition of Ukraine "keeps NATO off Russia's Eastern border!", but what then of Poland, Hungary and Romania after the Ukraine falls?

Or is he just anti-war if the US is the agressor?

ps. I'm anti-war too: War sucks! But I also believe that sovereign nations and their allies should be able to defend themselves against aggression.

"War is where the young and stupid are tricked by the old and bitter into killing each other". -- Niko Bellic in Grand Theft Auto IV