After the dropping of the first atomic bomb on Hiroshima, Japan on August 6, 1945, the Japanese still would not surrender according to the Potsdam Declaration. Kokura had been selected as the primary target with Nagasaki as a backup target for the plutonium based Fat Man bomb. Nagasaki was a major Japanese port and was very important to the Japanese war effort due to the production of ammunition, ships, military equipment and other war supplies. Nagasaki was primarily constructed of wooden buildings and, due to a lack of decent building codes, allowed buildings to be crowded together. Some of the inhabitants of Nagasaki had been evacuated before August 1945 because a few conventional bombs had been dropped on the city.
The bombing schedule was moved up from August 11, 1945, to August 9, 1945, because of a bad weather. On the morning of August 9, 1945, the B-29 Superfortress bomber Bockscar piloted by Major Charles W. Sweeney took off carrying the Fat Man bomb. After the Enola Gay and another B-29 flew over Kokura and reported cloud cover that would make bombing difficult, Nagasaki was chosen as the target for Fat Man and the Bockscar and its two escorts flew on to Nagasaki. If Nagasaki was also cloudy, Bockscar was going to divert to Okinawa and drop the bomb in the ocean to dispose of it.
When two B-29 Superfortresses were sighted in Nagasaki around 11 AM, no alarm was given because it was assumed that they represented a reconnaissance mission and no bombing threat. There were clouds but a break in the cloud cover allowed the bombardier to sight the target. The Fat Man bomb with about fourteen pounds of plutonium was dropped and exploded about fifteen hundred feet above the industrial district of Nagasaki. The bomb was about two miles off target. The explosion was equivalent to twenty one tons of TNT. Most of the destruction was confined to Urakami valley. Hills inside the city protected some of the other areas. Immediate casualties were around sixty thousand and ultimately over eighty thousand died by the end of 1945. The radius of total destruction was about one mile with fires and damage spreading out another mile.
Plans were made to manufacture and drop more atomic bombs on Japan if they did not surrender. There was a debate about whether each bomb should be dropped as soon as it was ready or whether they should be stocked piled and then dropped together in a short period. Several more Fat Man bomb assemblies were readied and plutonium cores were to be shipped to Tinian in the Marianas on August 12, 1945, three days after the dropping of the bomb on Nagasaki.
On August 9, 1945, the Japanese war council met and received the instruction from the Emperor to contacted the Allies and accept their terms for surrender. Part of the reason for the quick capitulation was the fact that the Russians had just declared war on Japan and had begun mobilizing for an invasion. One condition the Emperor requested was that his status as Supreme ruler not be challenged and this was apparently accepted by the allies. The Japanese officially surrendered to the Allies on August 10, 1945.
Nagasaki after bombing from nuclearfiles.org:
