Peconic, Long Island, New York  August 2, 1939

On this date in 1939, one month before the outbreak of World War II in Europe, German-born mathe­ma­tician and physi­cist Albert Ein­stein wrote Presi­dent Franklin D. Roose­velt about the possi­bility of using a nuclear chain reaction to pro­duce enor­mous amounts of energy that could be used in making a bomb. German scien­tists, he noted in closing his letter, were busy researching just that possi­bility, having formed a Uran­verein (Ura­nium Club) earlier in April as part of their coun­try’s clan­des­tine efforts to develop and produce nuclear wea­pons of war. Roose­velt did not receive Ein­stein’s confi­dential hand-de­liv­ered letter until Octo­ber 11, but when he did he acted on it quickly, appointing a com­mit­tee, the S‑1 Uranium Committee, to direct nuclear research starting with a $6,000 outlay.

In 1942 Italian-born Enrico Fermi and Hun­garian-born Leó Szi­lárd—col­leagues whom Ein­stein had men­tioned in his letter—went on to create the first atomic chain reaction at the Uni­ver­sity of Chi­cago and became mem­bers of the Man­hat­tan Engi­neering Dis­trict, the cover name for America’s atomic bomb pro­gram that evolved out of the S‑1 Com­mit­tee. (The project’s cover name derived from the then concen­tra­tion of nuclear energy exper­tise in Man­hattan, a New York City borough.) Leading the scien­tific side of the project was a young 38‑year-old Amer­i­can, J. Robert Oppen­heimer. Com­manding the project over­all was West Point graduate Brig. Gen. Leslie Groves.

Apart from the initial July 16, 1945, deto­na­tion of a nuclear (pluto­nium) bomb nick­named “Gadget” at the top of a 100‑ft tower in the New Mexico desert, Man­hat­tan Pro­ject’s deploy­able nuclear wea­pons were “Little Boy” (ura­nium bomb) and “Fat Man” (plu­to­nium bomb). (Szi­lárd and 154 nuclear scien­tists lost the moral argu­ment to invite Japa­nese obser­vers to view a second nuclear deto­na­tion, which they believed would have induced Japan’s leaders to sur­render unconditionally and thus spare lives.)

Initially six Japanese cities were identi­fied as can­di­dates for nuclear incin­er­ation. The list was later reduced to four: Hiro­shima on south­ern Hon­shū Island, the largest of the Japa­nese Home Islands; Kokura (Hiro­shima’s back­up) and Naga­saki (Kokura’s backup) on Kyūshū, the southern­most Home Island; and Nii­gata on north­ern Hon­shū. Weather that per­mitted visual bombing settled Hiro­shima’s terri­ble fate; clouds and smoke over Kokura from an earlier U.S. fire­bombing of Yawata (Yahata), less than 5 miles away, settled that of Naga­saki. On August 15, six days after Naga­saki and tens of thou­sands of deaths later, Emperor Hiro­hito (post­humously referred to as Emperor Shōwa) announced his country’s uncon­di­tional sur­render to the Allies. The Japanese Instrument of Surrender, signed on Septem­ber 2, 1945, aboard the U.S. battleship Missouri at anchor in Tokyo Bay, officially ended World War II.

Trinity Site, New Mexico, 1945: Detonating the First Nuclear Bomb

Trinity site test tower, New Mexico, July 1945 Explosives being readied for hoisting, Trinity site test tower, New Mexico, July 1945

Left: The 100-ft-tall tower constructed for the Trinity test. Trinity was the code­name of the world’s first deto­na­tion of a nuclear device, which occurred at the Alamo­gordo Bombing and Gun­nery Range (now part of the White Sands Missile Range) in Southern New Mexico on July 16, 1945, a date usually con­sidered to be the beginning of the Atomic Age.

Right: The explosives of the “Gadget” were raised up to the top of the tower for the final assembly in mid‑July 1945.

Bomb assembly group leader, Trinity site, New Mexico, July 15, 1945 Trinity test mushroom, New Mexico, July 16, 1945

Left: Norris Bradbury, bomb assembly group leader, stands next to the partially assembled “Gadget” atop the test tower, July 15, 1945.

Right: Trinity was a test of an implosion-design plutonium device, the same con­ceptual design used in the second nuclear device dropped on Japan, “Fat Man,” which was deto­na­ted over Naga­saki, Japan, on August 9, 1945. This photo was taken a 0.016 second after test detonation.

Aerial view of ground zero, New Mexico Oppenheimer and Groves at Trinity ground zero, New Mexico

Left: An aerial photograph of the Trinity crater shortly after the test. The nuclear device exploded with an energy equi­va­lent to around 20 kilo­tons of TNT and left a crater of radio­active glass 10 ft deep and 1,100 ft wide. The shock wave was felt over 100 miles away, and the mushroom cloud reached 7.5 miles in height.

Right: J. Robert Oppenheimer (center, in light-colored hat), Brig. Gen. Leslie Groves (to Oppen­heimer’s left), and others at ground zero of the Trinity test site some­time after the bombing of Hiroshima and Nagasaki.

History Channel’s Modern Marvels: Manhattan Project Documentary

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