So tell me, what exactly do you believe "After the election I will have more flexibility" being said to Medvedev in 2012 about a missile shield in Europe, has to do with Putin deciding to invade Ukraine in 2014?
In 2012, Yanukovych was still pretending to move towards closer ties with the EU, and the Maidan Revolution hadn't even occurred yet, Putin believed he was still on track for turning Ukraine into another Belarus.
Obama bears his portion of responsibility for what's going on now in Ukraine, as he should have leveled crippling sanctions against Russia then, but trying to pin it on him is nonsensical at best. Bush, Merkel, Schroeder, Sarkozy, Obama, Biden, and Trump all have contributed to this clusterf*ck, along with a significant portion of the GOP as a whole.