Vladimir Putin's Russia seized Ukraine's Crimean peninsula by force on the weekend and now has his sights on the rest of his Slavic neighbor. The brazen aggression brings the threat of war to the heart of Europe for the first time since the end of the Cold War. The question now is what President Obama and free Europe are going to do about it.

With a swiftness and organization that suggests the plans were hatched weeks ago, Mr. Putin is moving to carve up Ukraine after Russia's satrap in Kiev, former President Viktor Yanukovych,...