A few thoughts: The untested bomb was the uranium gun type, which they knew would work so they didn't have to test it. Fire a uranium plug into a uranium donut, and you have critical mass just by putting the two pieces together. The tested bomb was the plutonium compression type, and they had to test the compression mechanism at Alamagordo to see if it would indeed work to compress the plutonium sufficiently to achieve critical mass, and not have the plutonium squirt out one or more sides/holes. The US became a non-nuclear power after the Nagasaki drop, and didn't become a nuclear power again until more weapons were finished (this thread says that should have been as soon as later that month, my older sources said it didn't happen until the fall). Truman was bothered by killing all those kids. Was he bothered by the firebombing of other Japanese cities with incendiaries, designed to set fire to civilian homes and create firestorms? Once you make the moral leap that you're OK with firebombing tens of thousands of civilians, it doesn't really matter if it's done with one big bomb or thousands of smaller ones. Which is one reason I have trouble with the moral arguments against having used nuclear weapons in 1945. American military planners, and the FDR/Truman Administrations, had already shown that they were OK with mass strategic bombing and inflicting mass civilian casualties by conventional means. We didn't know how close the Japanese were to surrendering (or if they were even close at all), even after dropping the second bomb on Nagasaki. Minutes from the Japanese War Council on August 10 indicate that the Soviet invasion of Manchuria on August 8 had as much, if not more, to do with the Japanese (split) decision to surrender.