Andrew: it’s important to note that the fire bombings of Dresden and Tokyo were actually responsible for more deaths than the atomic blasts and were no more obviously defensible.
For what it’s worth, the firebombing of Dresden is estimated to have killed somewhere between 35,000 and 135,000 people, with most recent estimates tending towards the lower end of that range ([1]). The low-altitude firebombing of Tokyo on the night of March 9-10, 1945 killed somewhere between 80,000 and 100,000 civilians in one night ([2]). Both were, as far as I’m concerned, unspeakable atrocities, and both were more deadly than the nuclear attack on Nagasaki (about 74,000 dead). But the atomic bombing of Hiroshima, which killed about 140,000 people, remains (as far as I know) the largest deliberate massacre of civilians in the history of the world.