There are two main targets for nuclear weapons, counter-force, and counter-value. Counter force means destroying the military capacity of the target, including its nuclear arsenal. Counter value means targeting cities, population centres, and soft economic targets like commercial ports, the things the target values and is built to protect.

Having a counter force posture basically means you’re able and willing to carry out a first strike, this is an aggressive posture. Having a counter value posture basically means having a second strike posture, that you accept your own destruction, but intend to make the cost of any attack so high as to make any victory unattainable.

Second strike weapons must be ready at all times to launch within a few minutes (in the case of the US doctrine) or be portable to remote locations and difficult to track (Soviet doctrine) First strike weapons have the luxury of taking many hours to fuel up and launch, for example the Jupiter missiles stationed in Turkey. It was the presence of these missiles, who’s only practical purpose was a first strike, counter-force attack that precipitated the Cuban missile crisis.

Developing a credible defense against ballistic missiles represented an intention to move from a reactionary second-strike, counter value posture to an aggressive first strike counter force posture. Developing credible ABM weapons was seen as an escalatory move. This, plus the invention of MIRV warheads lead to the SALT and START treaties, where both sides recognised there were two possible outcomes - a never-ending path of escalation with ABM weapons and aggressive first strike posturing, or deliberately choosing not to develop ABMs, ensuring your own destruction if attacked, while adopting the unthinkable counter value posture, and settling into a lasting balance of power.

Ronald Reagan’s Star Wars bullshit then upended the balance of power a little bit, by claiming to be able to fully protect the US against incoming missiles, which if true would have made a first strike attack by the US survivable. The Regan administration actually intended to destabilize the Soviet economy by forcing them to adopt an identical posture, sinking billions of rubles into their own star wars program (like they did with the space race). Only, Regan fucked up because he didn’t consider there was another option. The Soviet doctrine wasn’t to match the US and eat itself alive while doing so, Soviet doctrine was to just nuke the star wars infrastructure if it ever became operational, and probably precipitate a full nuclear exchange in the process.

MIRVs disrupted the balance of power by enabling one missile to deploy enough decoys and warheads that even at a 90% kill rate, the 10% that made it through were enough counter-value to make any war unwinnable.

More powerful warheads were less desirable over just more of them. The development of MIRVs actually made START look more desirable, and so contributed to the balancing of power and easing of tensions. Whichever idiot decided to station Jupiters in Turkey nearly cost us everything.

Developing credible anti-nuc defences is a mugs game, it only ensures your adversary will come up with new and crazier ways to nuke you. Examples include the SLAM missile, an open cycle nuclear powered cruise missile, the Russian Posiden nuclear powered nuclear torpedo with unlimited range, cobalt bombs, nuclear landmines, all sorts of fun stuff.