Here's a little insight from Truman on the method of choosing the targets of Hiroshima and Nagasaki:
This weapon is to be used against Japan between now and August 10th. I have told the Sec. of War, Mr. Stimson, to use it so that military objectives and soldiers and sailors are the target and not women and children. Even if the Japs are savages, ruthless, merciless and fanatic, we as the leader of the world for the common welfare cannot drop that terrible bomb on the old capital or the newWe bombed legitimate targets in an effort to stop the war and avert a much worse (especially for the Japanese) invasion.
Dresden was a city of little industrial importance and cramped full of refugees fleeing the Russians at a time when Germany's industrial might had been crushed and forces were advancing on both sides. The war would be over in two more months.
The British firebombed an entire city of civilians and wounded soldiers for dubious military reasons, destroying 14,000 homes, 72 schools, 22 hospitals and other buildings, and apparently no military targets. To give an idea of the calculated devastation, the British first dropped HE bombs to expose the timbers of all of the buildings, then dropped incindiary bombs to light them, then more HE and strafing to hamper firefighting efforts. When the air above the city heated enough, cold air rushed in at ground level to replace the quickly rising hot air, sucking people into the firestorm. This as opposed to a simple HE bombing to take out military targets. The attack was designed to completely devastate a civilian population, and had a body count higher than that of Hiroshima.