All the president is doing is mild toothless sanctions, by all rights we should have went to war to protect our treaty obligations with the Ukraine and the UK. I am very glad the president didn't but Obama isn't poking Putin at all, more like bending over backwards to prevent war, not starting one. Putin called our bluff, it is as simple as that.
Also, if it were a totally conventional war, there is absolutely no war Russia could win against NATO. I mean, it isn't even in the realm of possibility. NATO could field 10 times as many troops and has better technology. Mr. Putin would be a fool to try and go to war, so he has to be confident that the NATO member countries are unwilling to get involved in a war. The threat really is, nuclear, because I don't see a strong man leader like Putin willing to lose if it did come to that.
The moral of the story is telling countries like Iran, don't give up your nuclear weapons. The Ukraine made an agreement with the United States, the UK to protect it's territory in exchange for giving it's nuclear weapons back to Russia, and we let them down. We are no longer the world's policeman and I am fine with that, but I don't think most people are really appreciating the consequences of this.