For the first time since the Soviet Union's collapse more than two decades ago, Russian military forces have moved into an Eastern European country and occupied its territory. Over 15,000 Russian soldiers are now stationed in Ukraine's autonomous republic of Crimea, according to Ukrainian officials (it's not clear how many of them were already in the region before this crisis), in a deployment ordered by Russian President Vladimir Putin to protect "Russian citizens and compatriots on Ukrainian territory." No shots have been fired, but Ukraine's acting president, Oleksandr Turchynov, has placed his country's military on its highest alert level to deter "potential aggression," as the United States condemned Russia's "invasion and occupation of Ukrainian territory" in violation of international law.
Fifteen independent countries, including Russia, emerged from the Soviet Union's disintegration. Six of them—Ukraine, Belarus, Moldova, and the three Baltic states of Estonia, Latvia, and Lithuania—are in Europe, and all of them have a complicated relationship with modern Russia. Seven other countries once belonged to the Warsaw Pact, the Soviet Union's military alliance in Eastern Europe. With the Cold War's end, none of them had faced the threat of military intervention by the communist superpower's successor state—until now. (In discussing Europe here, I'm not including Eurasian countries like Georgia, which fought a war with Russia in 2008, or the military support Russia offered Moldova's breakaway Transnistria region in the early 1990s.)