Who to blame for the Russian invasion of the Crimea

I often hear these days a constant refrain among liberals that Republicans put too much blame on president Obama for the actions of Putin. After all, it's the Russian strongman who invaded Crimea, not Barack Obama. I think this view is emotional and ahistorical. Vladimir Putin is a former KGB officer, a man with clear and unambiguous views on communism and Russian nationalism. He is a predator, a killer, a man who will take what he wants by force or deceit.  It is in his nature to attack the weak if it profits him. To blame Putin for the invasion into Ukraine is like blaming the fox for stealing your chickens. The person who is truly to blame is the man who let the barn open, who let Putin think  that he could take Crimea with few consequences. And that man is Obama, the president of the United States, the very person whose weak foreign policy made the Ukrainian invasion inevitable.