There's a distinction between defense and deterrence the former stops something that's been done, eg shooting down missiles and bombers, and the latter in the n

There's a distinction between defense and deterrence the former stops something that's been done, eg shooting down missiles and bombers, and the latter in the nuclear context is always maintaining the ability to strike back. A pre-emptive strike isn't defense it's just starting the war and it also doesn't prevent you from being struck. Unless you're in Cuba firing at the US or Europe aiming at Russia there's always time to launch a retaliatory strike before the first missiles hit and missile bases were designed to survive really close hits. That's part of why the US freaked out so much about missiles coming to Cuba and why Russia developed more mobile launch capability than the US. The only real 'defense' against and ICBM (at the time we're maybe figuring out real defenses now) was to be able to survive to strike back which isn't so much a defense as deterrent.