<P> The Japanese military was unable to stop the Allied attacks and the country's civil defense preparations proved inadequate . Japanese fighters and antiaircraft guns had difficulty engaging bombers flying at high altitude . From April 1945, the Japanese interceptors also had to face American fighter escorts based on Iwo Jima and Okinawa . That month, the Imperial Japanese Army Air Service and Imperial Japanese Navy Air Service stopped attempting to intercept the air raids in order to preserve fighter aircraft to counter the expected invasion . By mid-1945 the Japanese only occasionally scrambled aircraft to intercept individual B - 29s conducting reconnaissance sorties over the country, in order to conserve supplies of fuel . By July 1945, the Japanese had stockpiled 1,156,000 US barrels (137,800,000 l; 36,400,000 US gal; 30,300,000 imp gal) of avgas for the invasion of Japan . While the Japanese military decided to resume attacks on Allied bombers from late June, by this time there were too few operational fighters available for this change of tactics to hinder the Allied air raids . </P> <P> The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, and its theoretical explanation by Lise Meitner and Otto Frisch, made the development of an atomic bomb a theoretical possibility . Fears that a German atomic bomb project would develop atomic weapons first, especially among scientists who were refugees from Nazi Germany and other fascist countries, were expressed in the Einstein - Szilard letter . This prompted preliminary research in the United States in late 1939 . Progress was slow until the arrival of the British MAUD Committee report in late 1941, which indicated that only 5--10 kilograms of isotopically enriched uranium - 235 was needed for a bomb instead of tons of un-enriched uranium and a neutron moderator (e.g. heavy water). </P> <P> Working in collaboration with the United Kingdom and Canada, with their respective projects Tube Alloys and Chalk River Laboratories, the Manhattan Project, under the direction of Major General Leslie R. Groves, Jr., of the U.S. Army Corps of Engineers, designed and built the first atomic bombs . Groves appointed J. Robert Oppenheimer to organize and head the project's Los Alamos Laboratory in New Mexico, where bomb design work was carried out . Two types of bombs were eventually developed . Little Boy was a gun - type fission weapon that used uranium - 235, a rare isotope of uranium separated at the Clinton Engineer Works at Oak Ridge, Tennessee . The other, known as a Fat Man device (both types named by Robert Serber), was a more powerful and efficient, but more complicated, implosion - type nuclear weapon that used plutonium created in nuclear reactors at Hanford, Washington . A test implosion weapon, the gadget, was detonated at Trinity Site, on July 16, 1945, near Alamogordo, New Mexico . </P> <P> There was a Japanese nuclear weapon program, but it lacked the human, mineral and financial resources of the Manhattan Project, and never made much progress towards developing an atomic bomb . </P>

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