The Mutually Assured Destruction (MAD) was only feasible if full release is inevitable. If a single ICBM was released, this would result in retaliation. The retaliation isn't a single missile in response, it is full release. The initial salvo doesn't just hit the offending nation, it hits all our enemies. This results in responses from all the enemy nations. It results in a chain reaction that encompasses the entire populated world. After the destruction, there is a secondary salvo that releases after a month or so. This hits areas where survivors may have hidden from the initial attacks. This is done automatically and with out human intervention. The concept is simple, if we are going to be destroyed, then no one else will be allowed to survive either. It is a lose-lose scenario that guaranteed no nuclear weapons could ever be used against anyone without the destruction of the human civilization.