I am about to post an article from National Review by Andrew McCarthy that explains the history that has enabled Putin to make these aggressive moves in the region and how the US foreign policy played right into his hands.
McCarthy's thesis is a real stretch. Countries have been doing these things since before man first put chisel to stone tablet. Germany annexed the Sudetenland because it could, not because of any legal rationalizations. If the EU had hit Russia with serious sanctions, Putin would have backed down. The reality is that Putin acted because nobody moved to stop him - not the Ukrainians, not the EU, and not the US. We are not the only players on the chessboard. If the Ukrainians are not prepared to fight for their territory, how is it our problem? And I would extend that logic to the rest of NATO, which has been steadily cutting defense budgets and simultaneously expanding their welfare states while lowering their debts even as our government debt rises in leaps and bounds.