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This is a great question
and just to give a contrary answer, why do you think that we know more now than harry trueman new back then? some of the information surrounding the bombing of hiroshima and nagasaki is still classified. we only found out the Japanese tried to surrender in June 1945 because of the british official secrets act allowing that information to go public after 40 years. so there is probably a lot we still don't know about what happened. I personally believe the bomb was not necessary and we should have allowed the to surrender in June 1945 when they tried to the first time.
Great question! No, it isn't fair to "Monday-morning quarterback" this decision, nor is it even usually done with much historical accuracy involved in the criticism!
The atomic bombing decision was made in view of the situation known and experienced during the time of the decision-making. The primary factor was obviously the casualty estimates that were projected for a conventional troop invasion landing on mainland Japan (over a million U.S. casualties estimated). There were then many other factors taken into account, including the just-recent (in 1945) record of fanatically suicidal defenses of Japanese islands.
This same type of second-guessing is, unfortunately, due to today's political correctness, applied to the removal of ethnic Japanese from the U.S. west coast district by General DeWitt, after FDR's executive order of February 1942. At the time, the west coast was in a panic and the Japanese navy unquestionably had the ability to land invasion forces (which they did in the Aleutian isles off Alaska). 50,000 of the "internees" were NOT U.S. citizens and many thousands spoke no English. The removal decision was a military measure under emergency war conditions.