> Perry tells us that parity is “old thinking” because nuclear weapons can’t actually be used—the risk of uncontrollable and catastrophic escalation is too high. They are only good for threatening the enemy with nuclear retaliation.

This is an important point, which I believe is not necessarily subscribed to by all those in the military. In particular, I am greatly concerned that the use of tactical nuclear weapons—even very small ones on the scale of just 100t TNT or so—is far more likely to occur than a full scale strategic nuclear exchange, and that this could lead to escalation with tactical nuclear weapons of ever greater size, eventually leading to a far higher probability that strategic weapons are eventually used.

I believe that if you looked at the Top Secret plans produced by the Pentagon and the Russian General Staff you would find many that call for "limited" use of tactical nuclear weapons in response to an overwhelming attack by conventional forces. In addition there are proposals such as the "nuclear bunker buster" that was floated a few years back during the invasion of Afghanistan.

The great problem is that once the use of nuclear weapons of any sort becomes at all normalised, there is no upper limit to their destructive capability. The largest nuclear weapon ever detonated, Tsar Bomba, was a three stage thermonuclear device and had a yield of about 50 Mt TNT, but such a weapon could be easily scaled up simply by adding additional thermonuclear stages. Once nuclear weapons become actively used in any capacity the destructive capability of a nuclear-armed state is limited only by its restraint.

Consider the Allies' strategic bombing campaigns against Germany and Japan in WWII. The goal was to destroy the enemy's ability to make war and to force them to surrender; this involved killing civilians and military personnel, and destroying infrastructure and production capability. Note that killing civilans served to advance the principal goal of winning the war but was not an end in itself. If the war could be won with everything exactly the same except for fewer enemy casualties, I believe any sane commander with ethics remotely similar to my own would take that choice. In other words, inflicting enemy casualties has positive instrumental value but negative intrinsic value. In contrast, Hitler saw killing Jews and other populations as having positive intrinsic value, as a goal worth pursuing in and of itself.

If I had complete authority over a nation's nuclear weapons I would never, under any circumstances, actually use them. In all respects I would act as though I would respond to a nuclear attack in kind without hesitation, but that would be a lie. If I were faced with an overwhelming strategic attack and had the opportunity to respond with an attack of equal magnitude, I would not. Such an act would not change the outcome of my own nation, it would merely inflict the same on my enemy, which since I am not a psycopath has negative utility. At that point the war is already lost, the best one could do is minimise the destruction inflicted on humanity as a whole.

So I believe Perry assertion "nuclear weapons can't actually be used" is literally correct in the logical extreme. The problem is that deterrence requires the appearance of being ready to use them, and that is very difficult to fake. I think this underscores the importance of control mechanisms. Historically such mechanisms were opposed because they might put the weapons out of action when they need to be used, for example the Minuteman ICBM codes were set to all zeroes until 1977 and even today the captain of a British Vanguard-class submarine has the capability to launch without a code being transmitted from headquarters. This lends credence to the idea that these militaries intend to actually use such weapons in the event, not just appear to be prepared to use them. If you wanted to just fake the appearance of being prepared to use them it would be sufficient to plan not to distribute the right codes: the deterrence effect would not be lessened at all as the enemy would never risk you not being able to distribute the correct codes in the event.