Perception? Obama has no teeth. His foreign policy has been a debacle. Russia called him on his shit; Putin has been testing the waters for some time now.
And the US does not have to do everything, but for years, the very perception of US strength has been enough to keep a lot of people in line. Many countries have seen the weakening position of the US and are now acting on it.
It isn't just Russia. NK, China, Russia, Iran, Syria etc....they have all been acting like little shit rats for the least few years and all Obama has done is make empty "red-line" threats and speeches that really say nothing one way or the other.
The world is changing, probably for the worse, and I think a lot of it has roots in the weakening global US position both militarily and monetarily. I fully expect to see a MAJOR war at some point before I die, and I am not that young.