It will come either by Hillary's neocons going Russian Bear-baiting; or, if Trump gets elected, the neo-cons in the GOP establishment going bear-baiting, or Putin thinking Trump is now treating NATO like some sort of protection racket and thereby taking back the former Soviet Republics of Estonia, Latvia and Lithuania. Maybe Finland, too.
Donald Trump just made imbecilic statements that if certain NATO countries in Europe haven't paid enough money in his estimation to the United States for their collective defense, then he'll just sit right back and let the Russians roll right over them, should Russia follow through on a decision to invade certain NATO countries.
Now our parents and grandparents knew of a certain British Prime Minister who flew to Munich and returned with a pact between the UK and Hitler, proclaiming "Peace in out time!" Soon enough, Hitler took over Czechoslovakia and the following September, invaded Poland.
To thread the needle of avoiding World War 3, if that's even possible, the United States needs to STOP poking the Russian Bear, i.e., get out of the Ukraine, and at the same time reaffirm its treaty obligation or promise as a lead pipe guarantee that it will defend the European NATO countries against a Russian attack. That means, of course, that we need to redirect our War Department dollars (622 billion of them) so as to spend them wisely, and not waste them on boondoggles such as the F-35 Lardbucket: no way is that thing going to compete with Russia's new Sukhoi fighter jets.
But most importantly, we should be waging peace at the same time we keep our committments and keep strong enough to keep them, in order to maintain the peace.