What counts as a defensive strategic nuclear weapon, as opposed to an offensive one? The whole concept is based on the Mutually Assured Destruction idea that emerged out the early Cold War. It was crazy, because you had to convice your opponent you were prepared to destroy the world if he started a fight.

But MAD had great advantages. It was stable. It was robust. It did not require skillful generalship in a chaotic environment. It ran itself. It made both sides leery of doing anything that might be misinterpreted as a hostile act, and it made the very thought of a defensive measure come across as a preparation for aggression, which in turn made aggression less likely. It may have been crazy, but it got us though the Cold War without a nuclear exchange. Whatever you may think about MAD, it worked.

The idea behind MAD was that you built lots of cheap, simple, robust, even primitive missiles, and put in a go-nogo trigger on it. If enough of them survived an enemy first strike, you could destroy your enemy. All it took was to knock out a major proportion of his cities, very few warheads, actually, and he was punished.

The alternative idea was Kahn's "flexible nuclear response" theory, that we should prepare for every conceivable type of nuclear war, that we would have a suite of responses available to us and we could pick and choose the best, and the enemy would have no idea what we might do, complicating his planning. It would make nuclear war like other types of war. It could be "managed". Of course, no one had ever fought a nuclear war before, so no one had any clue how it would actually turn out in the confusion of actual combat. Like most theories of this type, it assumes we'll have knowledge of events and the ability to assess the situation and give orders, and have them carried out. In other words, that command and control, the chain of command, would remain intact during a general nuclear exchange.
This is incontrovertible evidence of stupidity of the highest order.

The ideal defensive weapon would be a cheap, durable, solid fuel booster in a deep silo, with a straightforward air burst warhead that could take out an enemy city even if it wasn't too accurate. Having lots of them would ensure the enemy couldn't take them all out in one massive preemptive strike.

Putting them on submarines was even better, because the enemy couldn't possibly take out all your subs at once. You could keep them in reserve and their skippers would launch only when they determined the war was indeed a real one (not an accident or false alarm).

With a triad of manned bomber, submarine, and silo missiles, it was highly unlikely an enemy could come up with a technical breakthrough that would neutralize all three. You were mutually assured enough ordnance would survive even the most massive enemy first strike to knock out a few dozen of his major cities and end his, and world, civilization.

First strike weapons were different. They had to be super accurate to take out hardened silos. Big warheads were not necessary, they just had to go off near the ground to take out the protected target. They also took advantage of MIRV concepts so a small number of missiles could delay detection and deliver a large number of weapons. Cruise weapons were also useful, they were slow, but much harder to detect by satellite, and cheap enough you could launch vast numbers of them to overwhelm any defense. Replacing our old clunky but dependable Minutemen with highly accurate MIRVed Peacekeepers in the same silos where the Russians knew exactly where they were was practically telling them "we have every intention of shooting first", and "we're going after your silos before you can empty them". This is not likely to encourage a "lets wait until we know for sure this is not a false alarm" attitude in your opponent. In fact, it makes for an itchy trigger finger and a high degree of instability. And the Russians were by no means innocent victims, they were doing he exact same thing we were.

Much of our nuclear strategy in the last half of the 20th century sure looked like a first-strike strategy to me, and this was not lost on the Russians. I knew the reason, we really didn't want to conquer the world, we just needed to keep our defense contractors busy building new, expensive weapons. It was Congressional and corporate inertia and greed, not empire-building megalomania.

But from a distance, the two are hard to tell apart.