Few people imagine the atomic bomb as an evolving weapons system, but in truth, that is what it is.
The first nuclear bombs were designed to be dropped by aircraft, but almost immediately they had to be reconfigured for the new Nazi technology, the ballistic missile. Missiles had a long way to go before they could carry major payloads, though, so long range bombers were still essential.
Once the Rosenberg and other Soviet spy rings had stolen US technology, the evolution of nuclear weapons changed again. This was because the Russians were obsessed with weapons size. They kept building larger and larger bombs until they had built one that would pretty much destroy a continent.
The US went in the other direction, with both its missiles and its bombs getting smaller, yet more powerful, through miniaturization.
Yet on top of this both the US an Russia started building smaller and smaller bombs, some that could be put in an artillery shell, with explosive force only about double that of a high explosive round.
From that, the emphasis was on putting multiple warheads on a single missile, and getting more and more yield from less nuclear material.
But since the time of Richard Nixon, as much or more emphasis has been put on anti-missile defenses. A nuke only works if you can get it to its target. And since Ronald Reagan, such weapons have become so good that much of the world’s missiles would be destroyed anyway.
The current state is the development of directed energy weapons that can shoot down missiles long before they are a threat. So nuclear weapons are no longer the ultimate weapon.
You’re dreaming if you think we have a fool proof SDI system.