Poland was meant to be the site for one of the European BMD installations. Since it would offset the nuclear mutually assured destruction balance that exists between Euro/US and Russia, Putin was very much against it. He said it would force the Russians to start amping up nuclear weapons armaments again. In an effort to avoid conflict and appease Moscow, the United States backed out of the agreement with Poland stating the planned site's in Romania and Turkey would have to be sufficient.
My point is, that since Russia is acting without regard for the United States's opinion on entering the Ukraine (knowing full well there is nothing the United States can do to stop the Russians), it would be a very opportune time to put the BMD site in Poland (knowing full well there is nothing Russia can do to stop the Americans).