Even though Putin lectured Obama at length, after forcing him to come to his dacha outside Moscow, about Russia's concerns, with a very special emphasis on Ukraine, the reality that the administration had to deal not with a nice ally and potential buddy but a tough guy with very clearly defined limits did not sink in.
In the end, the street triumphed over the elite. Ukrainian President Viktor Yanukovych tried to hang on to power, and failed. Russian President Vladimir Putin tried to maintain Russian influence, and failed. The EU tried to mediate, and failed. And the United States tried to... well, I'll get to that in a moment.