In the aftermath of what appears to be a settling of the South Ossetian war between Russia and Georgia, the US and Poland ratcheted up tensions another notch by Poland agreeing to accept a US missile interceptor base on its soil. Back in 1983, the original design for what was then quickly dubbed "Star Wars" (A continuation of the ABM system which had been temporarily forced to proceed as an entirely privatized system running "on spec") was for it to be a series of satellites that would launch small rockets upon detecting a missile launch. As that was rather obviously a system that could very easily be used in an offensive, first-strike role (A rocket fired from the upper atmosphere would be traveling quickly enough to wipe out just about anything without even the need to carry an explosive payload), it was reconfigured to use different means to knock down missiles.
In 2007, Russian President Vladimir Putin suggested that the US integrate its latest iteration of Star Wars, "Missie Defense," with a Russian weapons system, specifically by using their Gabala radar station located in Azerbaijan. The US countered that they could build safeguards into their system so that Russians need not be concerned over it being used against them. Obviously, for the US to have constructed an integrated system would have meant that the two systems could not have been un-integrated in a hurry. Looking at it from the Russian perspective, they clearly would have felt much calmer and more relaxed had their proposal been accepted.
The Russians have made it quite clear that they interpret the Polish acceptance of a Missile Defense system as a hostile move aimed at them. Apparently, America's neoconservatives have been trying to encircle Russia since at least the administration of the Elder George Bush. Of course, as US military assets are pretty thoroughly tied up in Iraq and Afghanistan, it's not at all clear that the US has any serious response available should Russia demonstrate its displeasure through more energetic means.