I said this below but Putin may not be so MAD. That is, he may feel destruction is not so mutually assured now. MAD was only really feasible when just three countries had nukes. It was abandoned a while back. Russia chose to pursue nuclear supremacy, over conventional defense. They have the biggest arsenal, as well as super-nuclear and super-sonic ICBMs to deliver them. Russia is the only country I know that thinks it might actually be able to win a nuclear war.