The single biggest threat, and I think this is a bigger threat than we want to believe, is Putin and his nukes. I think he could be pushed far enough to use one, I really do. I know that MAD is the deterrent, but I could see him betting that we won't retaliate for the very same reason. If he got a wild hair and nuked Berlin, that would be devastating, and frankly I am not sure we would retaliate as no one wants a global nuclear war, which is what that would precipitate. I think he knows this and would be willing to use one nuke to "send a message", and bet that we would not retaliate in kind because we want to avoid the fall-out from that even more.