There were three real reasons why the two atomic bombs were dropped on Japanese cities, and saving lives was not one of them.Taking away the cultural, religious, and most of all, the political reasons why both the US and Japan had little choice in the matter of ending the war, I find it difficult to understand why several million Americans and Japanese needed to die when a weapon was available that could end the fighting and posturing in a matter of days, and not months of what would have been the bloodiest fighting of WWII.
The reasons were:
1. To test the effectiveness of the new weapon - for that reason, both Hiroshima and Nagasaki had been spared conventional bombing, so that the effects of atomic bombing could be accurately measured.
2. To intimidate the Soviet Union.
3. To knock Japan out before the Soviet Union could come into the war and occupy Japanese territory.
At Yalta, the Soviet Union had promised to enter the war against Japan at the latest three months after the end of the war in Europe. The surrender of the German forces to the Soviet Union had occurred on 9 May, so the Soviet Union was due to attack Japan on 9 August.
By the end of July 1945, the US Government knew that the Soviet Union was about to attack Japan, in accordance with its Yalta promise. It also knew that Japan was on its last legs, and that the Red Army attacked, the Japanese forces would immediately crumble, allowing the red Army to sweep down through Manchuria and Korea and to invade Kyushu in the south, and to sweep down through Sakhalin and the Kurile Islands and invade Hokkaido in the north. The US Government needed to force Japan to surrender to the US alone, thereby depriving the Soviet Union of any excuse for invading, and allowing the US to be the sole occupying force (with token representation from the Western Allies). The US wanted Japan as an aircraft carrier for the coming war with the Soviet Union, then seen as inevitable by the US military.
The scenario foreseen by the US Government did come to pass. The Red Army attacked on 9 August, one day after the dropping of the bomb om Hiroshima, and the million-strong Kanto Army collapsed almost immediately. The Red Army swept to the south of Korea. It also occupied Southern Sakhalin and the Kurile Islands, and was on the point of landing on Hokkaido. If Japan had not surrendered at that point, the Red Army would have landed on Kyushu and Hokkaido and may well have occupied the whole of the country before US forces could get close to the place.
The rapid collapse of the Kanto Army and the occupation of Manchuria, Japan's last industrial base, and the occupation almost without opposition of Sakhalin and the southern Kuriles, which were regarded as part of the Japanese homeland, give the lie to the claim that there would have been fanatical resistance costing millions of casualties to any landing on the Japanese mainland. The partial extermination of the civilian population of Okinawa by the sub-humans of the US Marine Corps had probably extinguished the will to resist of the Japanese people.