The disposition of Crimea has an odd history. At some point after the Bolshevik Revolution, it was assigned to the Russian Federation, but in 1954 Khrushchev gave it to Ukraine. The majority of Crimea’s population, though, is Russian. And ever since the breakup of the Soviet Union, many Russians there and in Russia itself have called for its return to Russia. Ideally, the question of whether Crimea should belong to Ukraine or Russia, or be independent, should be decided by an internationally-monitored referendum. What Putin has done is highly provocative, but it also has support on the ground. The question now is whether he’ll go after other parts of eastern Ukraine where there are large Russian populations but where they are not a majority (Crimea is the only place in Ukraine where they are). Obama’s reaction so far has been underwhelming. On the other hand, I’m not sure what he can do.