Chances that the U.S. and Russia will clash militarily over Moscow’s invasion of Ukraine are very, very slim. Ukraine isn’t a member of NATO, and President Obama isn’t likely to volunteer for another war. But many of Ukraine’s neighbors are NATO members, including Poland, Romania, Slovakia, and Hungary. And so are the the Baltic states — Lithuania, Latvia, and Estonia — further north and right on Russia’s border.
If any of those countries come to Ukraine’s aid and find themselves in a war with Russia, NATO is obliged to intervene. That’s also true if Russia comes up with some pretext to invade any of those countries, unlikely as that seems. If we learned anything from World War I, it’s that huge, bloody conflicts can start with tiny skirmishes, especially in Eastern Europe.