World War II. By early 1939, only months before the start of World War II, physicists in the United States had become aware of the potential military applications of nuclear energy. They became concerned Nazi Germany might develop a nuclear weapon. In August 1939, the German-born physicist Albert Einstein helped alert U.S. President Franklin D. Roosevelt to the potential military applications of nuclear fission. World War II began on Sept. 1, 1939. The United States entered the war in December 1941. In 1942, the U.S. government set up the Manhattan Project to design and build a fission bomb.
On July 16, 1945, Manhattan Project exploded the first experimental nuclear device. The device, set off at the Trinity test site near Alamogordo, New Mexico, was a 22-kiloton implosion-type fission device. The test convinced U.S. leaders that fission weapons could be built.
The first nuclear weapon used by the United States against Japan was a gun-type fission bomb. It had a yield of about 13 kilotons. The bomb was dropped from a B-29 aircraft on the city of Hiroshima on Aug. 6, 1945. Three days later, another B-29 dropped a 22-kiloton implosion-type fission bomb on Nagasaki, Japan. These bombs largely destroyed both cities, but the number of deaths differed greatly. The smaller bomb killed from 70,000 to 100,000 people in Hiroshima, which has a flat terrain. The larger bomb killed about 40,000 in Nagasaki, which has a hilly terrain. Other people in both cities died later of injuries and radiation. On Aug. 14, 1945, Japan agreed to surrender, ending World War II.
Excerpt from the "Nuclear weapon" article, The World Book Encyclopedia © 1999