Everyone recognized that it was so dangerous to use them after the first two mass casualty events. At the time and even into the 50s it was not universally obvious, and the arguments in favor of nuclear weapons use were quite similar to arguments I often see with regards to AI: bombing cities into rubble is not a new concept, traditional explosives well within the supply capacity of large militaries are capable of it, so what are we even talking about when we say that there's scary new capabilities?