Game Theory: branch of mathematics used to calculate behavior in strategic situations.
Mutually Assured Destruction: a form of Nash Equilibrium, where if two opposing sides launch nuclear missiles at each other, they destroy themselves; by which, if one side threatens a first-strike launch, the other one threatens ability to retaliate *after* impact, so preventing combat.
MAD applied to the Cold War: If you think the other side is going to launch, you either launch first, or convince the enemy that you can strike back beyond any destruction they can do.
"Star Wars": Ronald Reagan, then-president of the US, invented missile defense: a group of space-set lasers and short-range missiles to intercept ICBMs. Not viable for massive missile launches.
Assuming that the american system would be incapable of dealing with a soviet first-strike, high Soviet officials decided that it was clearly designed to eliminate a soviet retaliation, and so were american preparations for a preemptive strike. They then designed, built and put it action the perfect retaliation weapon: Perimeter.
This system was so designed as to process information relative to the detonation of a nuclear missile on soviet territory, decide if it was a detonation or not, attempt to contact Soviet General Staff, and then proceed to send orders to launch to the direct person in charge of the launch, not any other person.
Now imagine, that Perimeter were still active. War breaks out between Russia (or any ex-soviet country) and any other country. The other country launches a missile. Or a missile accidentally detonates. Or a nuclear plant explodes. As the Soviet General Staff no longer exists, the order is sent. Some ballistic officer in a bunker someplace presses the button. Goodbye, USA.
Moscow considered it prudent to keep it.
It's still active.
