On August 6, 1945, an atomic bomb was dropped on Hiroshima, the primary target of the scheduled bombing. On August 9, 1945, an atomic bomb was dropped on Nagasaki which was the secondary target of the scheduled bombing. These atomic bombs killed either that day or in the ensuing weeks an estimated total of over 200,000 civilians. While the bombs did also kill Japanese soldiers and took out some infrastructure, the deliberate and premeditated annihilation of civilians on this scale was unprecedented.
History has told us that if not for these atomic bombs, Japan would not have willingly surrendered and therefore the continuation of the war would have resulted in the death of many thousands more of American soldiers in order to bring about Japanese capitulation. Logically, and without even a cursory review of historical documents, this doesn't make sense. Italy had surrendered on September 3, 1943, and Germany surrendered on May 7, 1945. Japan was challenged with a two-front war against the Russians by land and the Americans by sea, so the ending for Japan was already written in the cards. General Eisenhower stated in 1963: "The Japanese were ready to surrender and it wasn't necessary to hit them with that awful thing."
Additionally, there are two more important arguments in regards to the atomic bombings. First, there was only a three-day gap between the first atomic bomb and the second atomic bomb. That gap should have been considerably longer, which would have allowed more time for diplomatic channels to ascertain the willingness of the Japanese to surrender with terms that were acceptable to the United States. Because of the incredible and horrible destructive power of the atomic bomb, considerations to humanity alone should have necessitated a meaningful cooling off period before the second atomic bomb was dropped. One atomic bomb, in of itself, accomplished more than enough, there should therefore have been no rush to drop a second.
The second argument in regards to the atomic bombings is the targets of the strikes themselves. The following cities were either primary or secondary targets: Hiroshima, Kokura, and Nagasaki. Hiroshima had an estimated population of 300,000 civilians with an additional 43,000 soldiers. Kokura had an estimated population of 130,000 and Nagasaki had an estimated population of 240,000. While each of the cities had legitimate military reasons to be bombed, the nature of the bomb dropped was so in-discriminatory that the result could only be massive civilian casualties, and therefore none of these cities should have been designated as targets. Instead, the military should have considered striking against an abandoned Japanese military base, or one of the many small uninhabited islands that surround Japan.
Chillingly though, within this, there is another reason, perhaps the real reason, why not one but two atomic bombs were dropped. The first atomic bomb was a uranium fission bomb and the second atomic bomb was a plutonium fission bomb. Japan became, therefore, the de facto real world test site for these different atomic bombs; the civilians killed were victims, and the warning shot was made quite clearly to our future red menace, the Soviet Union.