why distinguish weapons of mass destruction?

Why distinguish between weapons of mass destruction and conventional weapons, since the latter can be much more destructive? (Compare a modern air bombing campaign with the use of sarin in the Tokyo Metro system, which killed just a handful of people). Some think that this distinction is simply a self-serving rule imposed by countries, such as the United States, that have tremendous advantages in conventional weaponry. But I think there is a good reason for the taboo on weapons of mass destruction (which has actually kept respectable nations from using them since Nagasaki). Human beings have a tendency to use dubious tactics past the point where they are justified. This happens in “arms race” situations, when each party uses its enemy’s behavior to justify doing a little bit worse in return. It also happens when one party reasons that x + 1 units of some dubious behavior are not much worse than x units, which would be OK. By this reasoning, one can gradually justify any amount of the questionable behavior.

Therefore, in general, we should prevent people from using tactics that have the potential to escalate out of control. For example, physical punishment can be much milder and more humane than imprisonment. But imprisonment has a natural limit (life without parole), which takes a long time to impose, so there is plenty of time to reconsider a draconian decision. Physical punishment, on the other hand, can quickly escalate to heinous torture. Thus it makes sense, in my view, to ban all physical punishment by governments. Likewise, each additional unit of destruction with conventional weapons costs extra money and takes more time and effort. Therefore, countries have to think hard before escalating a conflict with conventional weapons. So-called “weapons of mass destruction” can actually be rather mild in their effects. But, like physical punishment, they can easily, quickly, and cheaply escalate to horrifying levels. Hence the taboo on their use is sensible and should be preserved.