This could be the month that determines the success or collapse of Vladimir Putin's strategic plan for Russia. Even as shrewd Russian diplomacy runs rings around a stumbling White House on Syria, and as NSA revelations by Mr. Putin's honored guest Edward Snowden continue to strain U.S. ties with allies, the Russian president's imperial dream is hanging by a thread.

His problem is Ukraine, which since the early 1990s has resisted multiple attempts by Russia—some diplomatic, some subversive, some bellicose—to bring it back under...