Today, the decision to drop the atomic bombs is seen as a very momentous one. Historians argue about whether the bombs should have been dropped. At the time, however, there was very little argument. The bombs were a weapon that could help to shorten the war and so they were of course going to be used.
There were some who felt the bombs were not necessary. General MacArthur, for example, is said to have believed the Japanese would have surrendered if only the US would have promised not to depose the Emperor. However, the US was committed to a policy of unconditional surrender and would not do this. General Eisenhower is said to have believed that Japan was ready to surrender even without the bombs.
However, the arguments for dropping the bombs were overwhelming. It was clear to most people that the only way that the Japanese could be forced to surrender unconditionally was if the US invaded the home islands. Japan was already being devastated by American bombing attacks and the US had reached all the way to Okinawa. Still, the Japanese did not surrender. (They didn’t even surrender immediately after the first bomb and there were many in the military who still didn’t want to surrender). If the choice was between invading and dropping the bombs, President Truman saw little reason to invade and incur huge numbers of casualties. The desire to avoid these casualties and to end the war quickly was the major reason for dropping the bombs.
Casualties were a key factor. Truman had been informed by his military advisers that it was necessary to invade the Japanese mainland since the imperial army would fight on a suicidal scale to counter the American troops. Due to such a tenacity in fighting, a high casualty count was to be expected from the American forces. This estimation greatly influenced Truman in his decision-making, since the operationally-ready nuclear weapons could be used to force Japan to surrender without an invasion of the mainland and the incursion of high casualty numbers - they could be used to preserve American life.