Sunday, December 8, 2024

Atomic Bombings on Hiroshima and Nagasaki Part II

 Potsdam Declaration

The 16 July success of the Trinity Test in the New Mexico desert exceeded expectations. On 26 July, Allied leaders issued the Potsdam Declaration, which outlined the terms of surrender for Japan. The declaration was presented as an ultimatum and stated that without surrender, the Allies would attack Japan, resulting in "the inevitable and complete destruction of the Japanese armed forces and just as inevitably the utter devastation of the Japanese homeland". The atomic bomb was not mentioned in the communiqué.

On 28 July, Japanese papers reported that the declaration had been rejected by the Japanese government. That afternoon, Prime Minister Kantarō Suzuki declared at a press conference that the Potsdam Declaration was no more than a rehash (yakinaoshi) of the Cairo Declaration, that the government intended to ignore it (mokusatsu, "kill by silence"), and that Japan would fight to the end. The statement was taken by both Japanese and foreign papers as a clear rejection of the declaration. Emperor Hirohito, who was waiting for a Soviet reply to non-committal Japanese peace feelers, made no move to change the government position. Japan's willingness to surrender remained conditional on the preservation of the kokutai (Imperial institution and national polity), assumption by the Imperial Headquarters of responsibility for disarmament and demobilization, no occupation of the Japanese Home Islands, Korea or Formosa, and delegation of the punishment of war criminals to the Japanese government.

At Potsdam, Truman agreed to a request from Winston Churchill that Britain be represented when the atomic bomb was dropped. William Penney and Group Captain Leonard Cheshire were sent to Tinian, but LeMay would not let them accompany the mission.

Bombs

The Little Boy bomb, except for the uranium payload, was ready at the beginning of May 1945. There were two uranium-235 components, a hollow cylindrical projectile and a cylindrical target insert. The projectile was completed on 15 June, and the target insert on 24 July. The projectile and eight bomb pre-assemblies (partly assembled bombs without the powder charge and fissile components) left Hunters Point Naval Shipyard, California, on 16 July aboard the cruiser USS Indianapolis, and arrived on Tinian on 26 July. The target insert followed by air on 30 July, accompanied by Commander Francis Birch from Project Alberta. Responding to concerns expressed by the 509th Composite Group about the possibility of a B-29 crashing on takeoff, Birch had modified the Little Boy design to incorporate a removable breech plug that would permit the bomb to be armed in flight.

The first plutonium core, along with its polonium-beryllium urchin initiator, was transported in the custody of Project Alberta courier Raemer Schreiber in a magnesium field carrying case designed for the purpose by Philip Morrison. Magnesium was chosen because it does not act as a neutron reflector. The core departed from Kirtland Army Air Field on a C-54 transport aircraft of the 509th Composite Group's 320th Troop Carrier Squadron on 26 July, and arrived at North Field 28 July. Three Fat Man high-explosive pre-assemblies, designated F31, F32, and F33, were picked up at Kirtland on 28 July by three B-29s, two from the 393d Bombardment Squadron plus one from the 216th Army Air Force Base Unit, and transported to North Field, arriving on 2 August.

Hiroshima

Hiroshima during World War II

At the time of its bombing, Hiroshima was a city of industrial and military significance. A number of military units were located nearby, the most important of which was the headquarters of Field Marshal Shunroku Hata's Second General Army, which commanded the defense of all of southern Japan, and was located in Hiroshima Castle. Hata's command consisted of some 400,000 men, most of whom were on Kyushu where an Allied invasion was correctly anticipated. Also present in Hiroshima were the headquarters of the 59th Army, the 5th Division and the 224th Division, a recently formed mobile unit.  The city was defended by five batteries of 70 mm and 80 mm (2.8 and 3.1 inch) anti-aircraft guns of the 3rd Anti-Aircraft Division, including units from the 121st and 122nd Anti-Aircraft Regiments and the 22nd and 45th Separate Anti-Aircraft Battalions. In total, an estimated 40,000 Japanese military personnel were stationed in the city.

Hiroshima was a supply and logistics base for the Japanese military. The city was a communications center, a key port for shipping, and an assembly area for troops. It supported a large war industry, manufacturing parts for planes and boats, for bombs, rifles, and handguns. The center of the city contained several reinforced concrete buildings. Outside the center, the area was congested by a dense collection of small timber workshops set among Japanese houses. A few larger industrial plants lay near the outskirts of the city. The houses were constructed of timber with tile roofs, and many of the industrial buildings were also built around timber frames. The city as a whole was highly susceptible to fire damage. It was the second largest city in Japan after Kyoto that was still undamaged by air raids, primarily because it lacked the aircraft manufacturing industry that was the XXI Bomber Command's priority target. On 3 July, the Joint Chiefs of Staff placed it off limits to bombers, along with Kokura, Niigata and Kyoto.

The population of Hiroshima had reached a peak of over 381,000 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 340,000–350,000. Residents wondered why Hiroshima had been spared destruction by firebombing. Some speculated that the city was to be saved for U.S. occupation headquarters, others thought perhaps their relatives in Hawaii and California had petitioned the U.S. government to avoid bombing Hiroshima. More realistic city officials had ordered buildings torn down to create long, straight firebreaks. These continued to be expanded and extended up to the morning of 6 August 1945.

Bombing of Hiroshima

Hiroshima was the primary target of the first atomic bombing mission on 6 August, with Kokura and Nagasaki as alternative targets. The 393rd Bombardment Squadron B-29 Enola Gay, named after Tibbets's mother and piloted by Tibbets, took off from North Field, Tinian, about six hours' flight time from Japan, at 02:45 local time. Enola Gay was accompanied by two other B-29s: The Great Artiste, commanded by Major Charles Sweeney, which carried instrumentation, and a then-nameless aircraft later called Necessary Evil, commanded by Captain George Marquardt. Necessary Evil was the photography aircraft.

After leaving Tinian, the aircraft made their way separately to Iwo Jima to rendezvous with Sweeney and Marquardt at 05:55 at 2,800 meters (9,200 ft), and set course for Japan. The aircraft arrived over Hiroshima in clear visibility at 9,470 meters (31,060 ft). Parsons, who was in command of the mission, armed the bomb in flight to minimize the risks during takeoff. He had witnessed four B-29s crash and burn at takeoff, and feared that a nuclear explosion would occur if a B-29 crashed with an armed Little Boy on board. His assistant, Second Lieutenant Morris R. Jeppson, removed the safety devices 30 minutes before reaching the target area.

During the night of 5–6 August, Japanese early warning radar detected the approach of numerous American aircraft headed for the southern part of Japan. Radar detected 65 bombers headed for Saga, 102 bound for Maebashi, 261 en route to Nishinomiya, 111 headed for Ube and 66 bound for Imabari. An alert was given and radio broadcasting stopped in many cities, among them Hiroshima. The all-clear was sounded in Hiroshima at 00:05. About an hour before the bombing, the air raid alert was sounded again, as Straight Flush flew over the city. It broadcast a short message which was picked up by Enola Gay. It read: "Cloud cover less than 3/10th at all altitudes. Advice: bomb primary." The all-clear was sounded over Hiroshima again at 07:09.

At 08:09, Tibbets started his bomb run and handed control over to his bombardier, Major Thomas Ferebee. The release at 08:15 (Hiroshima time) went as planned, and the Little Boy containing about 64 kg (141 lb) of uranium-235 took 44.4 seconds to fall from the aircraft flying at about 9,400 meters (31,000 ft) to a detonation height of about 580 meters (1,900 ft) above the city. Enola Gay was 18.5 km (11.5 mi) away before it felt the shock waves from the blast.

Due to crosswind, the bomb missed the aiming point, the Aioi Bridge, by approximately 240 m (800 ft) and detonated directly over Shima Surgical Clinic. It released the equivalent energy of 16 ± 2 kilotons of TNT (66.9 ± 8.4 TJ). The weapon was considered very inefficient, with only 1.7 percent of its material fissioning. The radius of total destruction was about 1.6 kilometers (1 mi), with resulting fires across 11 km2 (4.4 sq. mi).

Enola Gay stayed over the target area for two minutes and was 16 kilometers (10 mi) away when the bomb detonated. Only Tibbets, Parsons, and Ferebee knew of the nature of the weapon; the others on the bomber were only told to expect a blinding flash and given protective goggles. "It was hard to believe what we saw", Tibbets told reporters, while Parsons said "the whole thing was tremendous and awe-inspiring ... the men aboard with me gasped 'My God'." He and Tibbets compared the shockwave to "a close burst of ack-ack fire".

Events on the ground

People on the ground reported a pika (ピカ)—a brilliant flash of light—followed by a don (ドン)—a loud booming sound. The experiences of survivors in the city varied depending on their location and circumstances, but a common factor in survivor accounts was a sense that a conventional weapon (sometimes cited as a magnesium bomb, which has a distinctively bright white flash) had happened to go off immediately in their vicinity, causing tremendous damage (throwing people across rooms, breaking glass, crushing buildings). After emerging from the ruins the survivors gradually understood that the entire city had been attacked at the same instant. Survivor accounts frequently feature walking through the ruins of the city without a clear sense of where to go, and encountering the cries of people trapped within crushed structures, or people with horrific burns. As the numerous small fires created by the blast began to grow, they merged into a firestorm that moved quickly throughout the ruins, killing many who had been trapped, and causing people to jump into Hiroshima's rivers in search of sanctuary (many of whom drowned). The photographer Yoshito Matsushige took the only photographs of Hiroshima immediately after the bombing. He described in a later interview that, immediately after the bombing, "everywhere there was dust; it made a grayish darkness over everything." He took five photographs in total before he could not continue: "It was really a terrible scene. It was just like something out of hell." Survivor accounts also prominently feature cases of survivors who appeared uninjured, but who would succumb within hours or days to what would later be identified as radiation sickness.

The exact number of people killed by the blast, firestorm, and radiation effects of the bombing is unknown. Difficulty in coming up with a correct figure is due to the imprecise record-keeping during the war, the chaos caused by the attack, lack of agreement on how many people were in the city on the morning of the attack, and uncertainty in methodology. Reports by the Manhattan Project in 1946 and the U.S. occupation–led Joint Commission for the Investigation of the Atomic Bomb in Japan in 1951 estimated 66,000 dead and 69,000 injured, and 64,500 dead and 72,000 injured, respectively, while Japanese-led reconsiderations of the death toll in the 1970s estimated 140,000 dead in Hiroshima by the end of the year. Estimates also vary on the number of Japanese military personnel killed. The United States Strategic Bombing Survey estimated in 1946 that there were 24,158 soldiers present in Hiroshima at the time of the attack and that 6,789 were killed or missing as a result; the 1970s reconsiderations estimated about 10,000 military dead. A modern estimate by the Radiation Effects Research Foundation (RERF) estimates a city population of 340,000 to 350,000 at the time of the bombing, of which 90,000 to 166,000 died by the end of the year.

U.S. surveys estimated that 12 km2 (4.7 sq. mi) of the city were destroyed. Japanese officials determined that 69 percent of Hiroshima's buildings were destroyed and another 6 to 7 percent damaged. Some of the reinforced concrete buildings in Hiroshima had been very strongly constructed because of the earthquake danger in Japan, and their framework did not collapse even though they were fairly close to the blast center. Since the bomb detonated in the air, the blast was directed more downward than sideways, which was largely responsible for the survival of the Prefectural Industrial Promotional Hall, now commonly known as the Genbaku (A-bomb) dome, which was only 150 m (490 ft) from ground zero (the hypocenter). The ruin was named Hiroshima Peace Memorial and was made a UNESCO World Heritage Site in 1996 over the objections of the United States and China, which expressed reservations on the grounds that other Asian nations were the ones who suffered the greatest loss of life and property, and a focus on Japan lacked historical perspective.

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