The United States detonated two nuclear weapons over the Japanese cities of Hiroshima and Nagasaki on August 6 and 9, 1945, respectively, with the consent of the United Kingdom, as required by the Quebec Agreement. The two bombings killed between 129,000 and 226,000 people, most of whom were civilians, and remain the only use of nuclear weapons in armed conflict.
The question of military necessity can be quickly put to rest. “Japan was already defeated and dropping the bomb was completely unnecessary.” They are the words of Dwight D. Eisenhower, Supreme Commander of the Allied Forces in Europe and future president of the United States. Eisenhower knew, as did the entire senior U.S. officer corps, that by mid 1945 Japan was defenseless.
But if dropping the bombs was not driven by military needs, why, then, were they used? The answer can be discerned in the U.S. attitude toward the Russians, the way the War ended in Europe, and the situation in Asia.
So, on August 6, 1945, two days before the Russians were to declare war against Japan, the U.S. dropped the bomb on Hiroshima. There was no risk to U.S. forces then waiting for a Japanese response to the demand for surrender. The earliest planned invasion of the island was still three months away and the U.S. controlled the timing of all military engagements in the Pacific. But the Russian matter loomed and drove the decision on timing. So, only three days later, the U.S. dropped the second bomb on Nagasaki. The Japanese surrendered on August 14, 1945, eight days after the first bomb was dropped.
Atomic bombings of Hiroshima and Nagasaki
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progressiveissuesblog 
Your comments are interesting. Is it possible to get some reference documentation?
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More research is needed into the one certain and lasting result of the atomic bombings of Japan: it is the one source of data on the effects of a nuclear weapon in a real-world, non-experimental setting.
In July 1944, Capt. William S. Parsons, head of the Ordnance Division for the Manhattan Project, led a team of scientists and engineers to Port Chicago to study “the effects of the detonation” after the massive blast that destroyed two ships and killed 320 Americans. (A local newspaper reported “Most Victims Atomized.”)
Parsons, trained as an Experimental Officer, served as technical director for Operation Crossroads, the massive and highly-publicized tests of atomic bombs against naval ships held in the Bikini Atoll (Marshall Island) in July 1946.
Speaking as Rear Admiral (the first “atomic admiral”) in 1948, Parsons told the Naval War College that the data from Port Chicago was vital to the research and development of the atomic bomb, and he went on to report that “The results of Halifax and Port Chicago have been confirmed on a larger scale by Hiroshima, Nagasaki, and Bikini “ABLE”. From these we can conclude that no spectacular blast damage will be done to tough structures like bridges, steel buildings, and heavily reinforced concrete buildings.”
(That conclusion applied to the atomic bombs produced at Los Alamos, which were like firecrackers in comparison to today’s thermonuclear weapons.) The important takeaway here is that more needs to be said about the how the lasting impact of early scientific data on nuclear policy.
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