That's the point of MAD. It's a Mexican Standoff, if anyone pulls the nuclear trigger then everyone does and everyone dies. That doesn't mean you can win by other means, any action that threatens the other party will be met with the nuclear option. The only players allowed to move or be moved upon are those outside of the game. Treaties like NATO can bring non-nuclear powers into the fold which serves to "protect" them but also signs them up for the suicide pact.