Bernard Brodie, naval strategist, figured this all out as the inevitable endpoint of nuclear weapons in 1946.
People have been trying to get around it ever since, but there is no way around it. Disarm, and a handful of nukes rule the world. If armed, the only way to avoid war is to make it utterly irrational.
And offence has an apparently permanent advantage over defence with MIRVs, as it will cost more to shoot down a rocket than to launch it, so missile defence doesn’t work as a strategy to exit MAD.
https://en.m.wikipedia.org/wiki/Bernard_Brodie_(military_str...
But not shooting a missile down is massively, massively more expensive. If the missile scare on Hawaii had been real and North Korea had launched a single nuclear missile and the US had shot it down, do you think anyone would have cared about the cost?
Even if it is, say, twice the cost to shoot down a missile as to launch it the US can afford more than twice the cost the Russians and much more than North Korea.
But it isn’t 2x. Try 10-20x. Modern MIRV’s can have ten warheads, and it is also substantially more difficult to target a moving missile than to hit a launch silo. Tests so far have not been anywhere near the accuracy needed to confidently guarantee you could shoot down a first strength. You need 100% performance, they’re getting less than 50%, for ideal conditions.
The economics just don’t work with a 20x cost gap. What’s more the attempt to built a unilateral defence system would encourage a massive arms race.
https://en.m.wikipedia.org/wiki/Multiple_independently_targe...
This drives the need to be able to respond.