Putin is right about the missile defense system being justified as protecting Europe from Iranian missiles. With the Iranian nuclear program halted it does not seem to have a purpose in that regard. But these things have inertia, they do not get started or halted quickly. That said, the ABM treaty was dissolved in 2002 by Bush, so the US is not breaking any treaty obligations. Obviously Putin does not like to see his nuclear missile threat degraded and Russia has always been paranoid about its national defense and it is never wise to poke the Bear too hard, it might react in negative ways. But Putin is not making anyone feel secure and happy on our side either with his taking of Crimea and support for Ukrainian separatists and his talk of nuclear war. So I think this may be a good way to get him back to the negotiating table. Dunno.