This was gotten from a previous wikiquestion answered by RoyR, this is his response.

The bombs, secretly developed by the United States (with assistance from the United Kingdom) under the codename Manhattan Project, were the second and third atomic bombs to be exploded, and are the only ones ever used as weapons, rather than for testing purposes. (The first nuclear test explosion, dubbed "Trinity," was conducted in a desert in New Mexico on July 16, 1945.) The decision to drop the bombs was made by US President Harry S. Truman, and followed over 3� years of direct involvement of the US in World War II, during which time the United States had suffered about 400,000 casualties. Truman's officially stated intention in ordering the bombings was to bring about a quick resolution of the war by inflicting destruction, and instilling fear of further destruction, that was sufficient to cause Japan to surrender. As detailed near the end of this article, whether or not the bombings were justified has long been a contentious issue.

Some believe the bombs were dropped as a warning to the USSR, who was just beginning to enter the war in Asia, as the prospect of a difficult post-war situation were becoming obvious in the discussions between the USA and Josef Stalin about the fate of Europe. Another theory suggests that the bombs were designed to prevent the USSR from invading Japan and being able to further her sphere of influence. It was another part of America's "containment" plan, some say.

Choice of targets

The Target Committee at Los Alamos on May 10-11, 1945, selected in order the following targets: Kyoto, Hiroshima, Yokohama, Kokura arsenal, Niigata, and possibly the Emperor's Palace. According to Robert Jungk, page 178:

"On the short list of targets for the atom bomb, in addition to Hiroshima, Kokura and Niigata, was the Japanese city of temples, Kyoto. When the expert on Japan, Professor Edwin O. Reischauer, heard this terrible news, he rushed into the office of his chief, Major Alfred MacCormack, in a department of the Army Intelligence Service. The shock caused him to burst into tears. MacCormack, a cultivated and humane New York lawyer, thereupon managed to persuade Secretary of War Stimson to reprieve Kyoto and have it crossed off the black list."

This account is partially confirmed by Rhodes, page 640, where he describes Stimson's refusal to bomb Kyoto, against the preference of General Groves.

Hiroshima during World War II

At the time of its bombing, Hiroshima was a city of considerable military significance. It contained the headquarters of the Fifth Division and Field Marshal Hata's 2nd General Army Headquarters, which commanded the defense of all of southern Japan. The city was a communications center, a storage point, and an assembly area for troops. It was chosen as a target because it had not suffered damage from previous bombing raids, allowing an ideal environment to measure the damage caused by the atomic bomb. The city was mobilized for "all-out" war, with thousands of conscripted women, children and Koreans working in military offices, military factories and building demolition and with women and children training to resist any invading force.

The center of the city contained a number of reinforced concrete buildings as well as lighter structures. Outside the center, the area was congested by a dense collection of small wooden workshops set among Japanese houses; a few larger industrial plants lay near the outskirts of the city. The houses were of wooden construction with tile roofs. Many of the industrial buildings also were of wood frame construction. The city as a whole was highly susceptible to fire damage.

The population of Hiroshima had reached a peak of over 380,000 earlier in the war but prior to the atomic bombing the population had steadily decreased because of a systematic evacuation ordered by the Japanese government. At the time of the attack the population was approximately 255,000. This figure is based on the registered population, used by the Japanese in computing ration quantities, and the estimates of additional workers and troops who were brought into the city may not be highly accurate.

The Bombing

Hiroshima was the primary target of the first U.S. nuclear attack mission, on August 6, 1945. The weather was good, and the crew and equipment functioned properly. Navy Captain William Parsons armed the bomb during the flight. (It had been left unarmed to minimize the risks during takeoff.) In every detail, the attack was carried out exactly as planned, and the bomb, with a 60 kg core of uranium-235, performed precisely as expected.

About an hour before the bombing, the Japanese early warning radar net detected the approach of some American aircraft headed for the southern part of Japan. The alert had been given and radio broadcasting stopped in many cities, among them Hiroshima. The planes approached the coast at a very high altitude. At nearly 08:00, the radar operator in Hiroshima determined that the number of planes coming in was very small�probably not more than three�and the air raid alert was lifted. The normal radio broadcast warning was given to the people that it might be advisable to go to shelter if B-29s were actually sighted, but no raid was expected beyond some sort of reconnaissance. At 08:15, the B-29 Enola Gay, piloted and commanded by Colonel Paul Tibbets, dropped the nuclear bomb called "Little Boy" over the central part of the city and it exploded about two thousand feet above the city with a blast equivalent to 13 thousand tons of TNT, killing an estimated 80,000 civilians outright.

Japanese Realization of the Bombing

The Tokyo control operator of the Japanese Broadcasting Corporation noticed that the Hiroshima station had gone off the air. He tried to re-establish his program by using another telephone line, but it too had failed. About twenty minutes later the Tokyo railroad telegraph center realized that the main line telegraph had stopped working just north of Hiroshima. From some small railway stops within ten miles (16 km) of the city came unofficial and confused reports of a terrible explosion in Hiroshima. All these reports were transmitted to the Headquarters of the Japanese General Staff.

Military headquarters repeatedly tried to call the Army Control Station in Hiroshima. The complete silence from that city puzzled the men at Headquarters; they knew that no large enemy raid could have occurred, and they knew that no sizeable store of explosives was in Hiroshima at that time. A young officer of the Japanese General Staff was instructed to fly immediately to Hiroshima, to land, survey the damage, and return to Tokyo with reliable information for the staff. It was generally felt at Headquarters that nothing serious had taken place, that it was all a terrible rumor starting from a few sparks of truth.

The staff officer went to the airport and took off for the southwest. After flying for about three hours, while still nearly 100 miles (160 km) from Hiroshima, he and his pilot saw a great cloud of smoke from the bomb. In the bright afternoon, the remains of Hiroshima were burning.

Their plane soon reached the city, around which they circled in disbelief. A great scar on the land, still burning, and covered by a heavy cloud of smoke, was all that was left. They landed south of the city, and the staff officer immediately began to organize relief measures, after reporting to Tokyo.

Tokyo's first knowledge of what had really caused the disaster came from the White House public announcement in Washington, sixteen hours after the nuclear attack on Hiroshima.

By the end of 1945, it is estimated that 60,000 more people died due to radiation poisoning, bringing the total killed in Hiroshima in 1945 to 140,000. Since then several thousand more people have died of radiation-related causes. (According to the city of Hiroshima, as of August 6, 2004, the cumulative death toll of atomic-bomb victims was 237,062, but it remains uncertain how many of them exactly died of the effects of the bombing. There are about 270,000 hibakusha, "A-bomb victims," still living in Japan.)

"Survival" of some structures

Some of the reinforced concrete buildings in Hiroshima were of a far stronger construction than is required by normal standards in America, because of the earthquake danger in Japan. This exceptionally strong construction undoubtedly accounted for the fact that the framework of some of the buildings that were fairly close to the center of damage in the city did not collapse. Another is that the blast was more downward than sideways; this has much to do with the "survival" of the Prefectural Industrial Promotional Hall (pictured), designed and built by the Czech architect Jan Letzel, which was only a few meters from ground zero. (The ruin was named Hiroshima Peace Memorial and made a UNESCO World Heritage site in 1996 over the objections of the US and China.

Nagasaki During World War II

The city of Nagasaki had been one of the largest sea ports in southern Japan and was of great wartime importance because of its wide-ranging industrial activity, including the production of ordnance, ships, military equipment, and other war materials.

In contrast to many modern aspects of Nagasaki, the residences almost without exception were of old-fashioned Japanese construction, consisting of wood or wood-frame buildings, with wood walls (with or without plaster), and tile roofs. Many of the smaller industries and business establishments were also housed in buildings of wood or other materials not strong enough to withstand explosions. Nagasaki had been permitted to grow for many years without conforming to any definite city zoning plan; residences were erected adjacent to factory buildings and to each other almost as closely as possible throughout the entire industrial valley.

Nagasaki had never been subjected to large-scale bombing prior to the explosion of a nuclear weapon there. On August 1, 1945, however, a number of high-explosive bombs were dropped on the city. A few of these bombs hit in the shipyards and dock areas in the southwest portion of the city. Several of the bombs hit the Mitsubishi Steel and Arms Works and six bombs landed at the Nagasaki Medical School and Hospital, with three direct hits on buildings there. While the damage from these few bombs were relatively small, it created considerable concern in Nagasaki and a number of people - principally school children - were evacuated to rural areas for safety, thus reducing the population in the city at the time of the nuclear attack.

The Bombing

On the morning of August 9, 1945, the crew of the American B-29 Superfortress, "Bockscar," flown by Major Charles W. Sweeney and carrying the nuclear bomb nicknamed, "Fat Man," found their primary target, Kokura, to be obscured by cloud. After three runs over the city and having fuel running low due to a fuel-transfer problem, they headed for their secondary target, Nagasaki. At about 07:50 Japanese time, an air raid alert was sounded in Nagasaki, but the "all clear" signal was given at 08:30. When only two B-29 superfortresses were sighted at 10:53 the Japanese apparently assumed that the planes were only on reconnaissance and no further alarm was given. A few minutes later, at 11:00, the observation B-29 ("The Great Artiste" flown by Capt. Frederick C. Bock) dropped instruments attached to three parachutes.

At 11:02, a last minute break in the clouds over Nagasaki allowed Bockscar's bombardier, Capt. Kermit Beahan, to visually sight the target as ordered. The weapon, containing a core of 8 kg of plutonium-239, was dropped over the city's industrial valley. It exploded 1,540 feet (469 m) above the ground almost midway between the Mitsubishi Steel and Arms Works, in the south, and the Mitsubishi-Urakami Ordnance Works (Torpedo Works), in the north, the two principal targets of the city. Some 75,000 of Nagasaki's 240,000 residents were killed, followed by the death of at least as many from resulting sickness and injury. However another report issues a different residential number, speaking of Nagasaki's population which dropped in one split-second from 422,000 to 383,000, thus 39,000 were killed, over 25,000 were injured. If taken into account those who died from radioactive materials causing cancer, the total number of residents killed is believed to be at least 100,000.

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