Whether justified or not, there were a lot of "reasons" for the use of the bomb that have been forgotten or are not adequately understood today. Just for starters:
- Many of the Manhattan Project scientists were motivated to build the bomb before the Nazis. Indeed, this seems to be one of the single biggest reasons for participation in the project. When it finally came out that the Germans were far behind and posed no real threat, a significant number of scientists quit entirely.
- The Soviets were on the brink of invading Japan and one of the "reasons" for the use of the atomic weapons was as a show of force / deterrent against the Soviets, in anticipation of the upcoming Cold War.
- The timeline was longer than I had known. Little Boy was dropped on August 6. Fat Man was dropped 3 days later, the same day that the Soviets declared war on Japan. The Japanese didn't surrender until August 15, six days later. The book mentioned that many historians think the Japanese were motivated to surrender more by the Soviet threat of invasion than by the atomic bombings.
In any case, it's a fascinating story that I think any technically-minded person would enjoy reading.