Well our leaders have quite the precident that I am afraid they are going to follow?
(Please don't take that wrong way Berlin, My Father is from Germany, but like I said my post, Putin will hit hard and then Compromise a little of position (Western Ukraine) therefore making our side feel like we gained something when in reality we lost East Ukraine.)
Also Excellent point about Siberia and China. In WWII Stalin was terrified that the Japanese would attack him in his rear. And during the tenuous time in '41 and '42 doing so would have spelled disaster for the Soviet Union (The reason the Japanese didn't, is because I think they would terrified of the Soviets too, but in retrospect they SHOULD have)