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...In the final year of
World War II, the
Allies prepared for a costly
invasion of the Japanese mainland. This undertaking was preceded by a
conventional bombing and firebombing campaign that devastated 64 Japanese cities. The
war in the European theatre concluded when Germany
surrendered on 8 May 1945, and the Allies turned their full attention to the
Pacific War. By July 1945, the Allies'
Manhattan Project had produced two types of atomic bombs: "
Little Boy", an
enriched uranium gun-type fission weapon, and "
Fat Man", a
plutonium implosion-type nuclear weapon. The
509th Composite Group of the
United States Army Air Forces was trained and equipped with the specialized
Silverplate version of the
Boeing B-29 Superfortress, and deployed to
Tinian in the
Mariana Islands. ...
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Hansell's successor,
Major General Curtis LeMay, assumed command in January 1945 and initially continued to use the same precision bombing tactics, with equally unsatisfactory results. The attacks initially targeted key industrial facilities but much of the Japanese manufacturing process was carried out in small workshops and private homes.
[37] Under pressure from
United States Army Air Forces (USAAF) headquarters in Washington, LeMay changed tactics and decided that low-level
incendiary raids against Japanese cities were the only way to destroy their production capabilities, shifting from precision bombing to
area bombardment with incendiaries.
[38] Like most
strategic bombing during World War II, the aim of the air offensive against Japan was to destroy the enemy's war industries, kill or disable civilian employees of these industries, and
undermine civilian morale.
[39][40]
Over the next six months, the XXI Bomber Command under LeMay firebombed 64 Japanese cities.
[41] The
firebombing of Tokyo, codenamed
Operation Meetinghouse, on 9–10 March killed an estimated 100,000 people and destroyed 41 km2 (16 sq mi) of the city and 267,000 buildings in a single night. It was the deadliest bombing raid of the war, at a cost of 20 B-29s shot down by flak and fighters.
[42] By May, 75 percent of bombs dropped were incendiaries designed to burn down Japan's "paper cities". By mid-June, Japan's six largest cities had been devastated.
[43] The end of the
fighting on Okinawa that month provided airfields even closer to the Japanese mainland, allowing the bombing campaign to be further escalated. Aircraft flying from Allied
aircraft carriers and the
Ryukyu Islands also regularly struck targets in Japan during 1945 in preparation for Operation Downfall.
[44] Firebombing switched to smaller cities, with populations ranging from 60,000 to 350,000. According to
Yuki Tanaka, the U.S. fire-bombed over a hundred Japanese towns and cities.
[45]
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The Target Committee nominated five targets:
Kokura (now
Kitakyushu), the site of one of Japan's largest munitions plants;
Hiroshima, an embarkation port and industrial center that was the site of a major military headquarters;
Yokohama, an urban center for aircraft manufacture, machine tools, docks, electrical equipment and oil refineries;
Niigata, a port with industrial facilities including steel and aluminum plants and an oil refinery; and
Kyoto, a major industrial center. The target selection was subject to the following criteria:
- The target was larger than 4.8 km (3 mi) in diameter and was an important target in a large city.
- The blast wave would create effective damage.
- The target was unlikely to be attacked by August 1945.[73]
These cities were largely untouched during the nightly bombing raids, and the Army Air Forces agreed to leave them off the target list so accurate assessment of the damage caused by the atomic bombs could be made. Hiroshima was described as "an important army depot and port of embarkation in the middle of an urban industrial area. It is a good radar target and it is such a size that a large part of the city could be extensively damaged. There are adjacent hills which are likely to produce a focusing effect which would considerably increase the blast damage. Due to rivers it is not a good
incendiary target."
[73]
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Hiroshima was the primary target of the first atomic bombing mission on 6 August, with Kokura and Nagasaki as alternative targets. The 393d 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.
[126] 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.
[127]
Special Mission 13, primary target Hiroshima, 6 August 1945
[127][128]
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),
[129] and set course for Japan. The aircraft arrived over the target in clear visibility at 9,470 meters (31,060 ft).
[130] 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.
[131] His assistant,
Second Lieutenant Morris R. Jeppson, removed the safety devices 30 minutes before reaching the target area.
[132]
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Bombing of Nagasaki

The
Bockscar B-29 and a post war Mk III nuclear weapon painted to resemble the Fat Man bomb, at the National Museum of the United States Air Force, Dayton, Ohio
Responsibility for the timing of the second bombing was delegated to Tibbets. Scheduled for 11 August against Kokura, the raid was moved earlier by two days to avoid a five-day period of bad weather forecast to begin on 10 August.
[188] Three bomb pre-assemblies had been transported to Tinian, labeled F-31, F-32, and F-33 on their exteriors. On 8 August, a dress rehearsal was conducted off Tinian by Sweeney using
Bockscar as the drop airplane. Assembly F-33 was expended testing the components and F-31 was designated for the 9 August mission.
[189]
Special Mission 16, secondary target Nagasaki, 9 August 1945[190]
At 03:47 Tinian time (GMT+10), 02:47 Japanese time,
[191] on the morning of 9 August 1945,
Bockscar, flown by Sweeney's crew, lifted off from
Tinian island with the Fat Man, with Kokura as the primary target and Nagasaki the secondary target. The mission plan for the second attack was nearly identical to that of the Hiroshima mission, with two B-29s flying an hour ahead as weather scouts and two additional B-29s in Sweeney's flight for instrumentation and photographic support of the mission. Sweeney took off with his weapon already armed but with the electrical safety plugs still engaged.
[192]
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en.wikipedia.org
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Other than the test in the New Mexico dessert, extent and effect of atomic bomb was varied calculations and speculations. Reported a few scientists on the project thought there might be a chance the detonation would spark a chain reaction across the whole planet.
As pointed out above, a handful of target cities were left out of the larger bomber raids so that full effects of the two types of bombs could be measured/gauged.
Bomb production in the USA was still at a trickle rate, about one bomb every few weeks and slowly increasing in ones and two over the next several months, so a "rain of atom bombs" would have take most of a year in time. And if Japan didn't surrender after the first two(three), the Allies likely would have been engaged in the mainland invasion of the Home Islands.