Peconic, Long Island, New York · August 2, 1939

On this date in 1939, one month before the outbreak of World War II, 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. 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 the 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. 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’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, then four: Hiro­shima on south­ern Hon­shū Island, Kokura (Hiro­shima’s back­up) and Naga­saki (Kokura’s backup) on Kyūshū, and Nii­gata on north­ern Hon­shū. Weather that per­mitted visual bombing settled Hiro­shima’s terri­ble fate; clouds and smoke over Ko­kura from an earlier 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, Japan announced its uncon­di­tional sur­render to the Allies, signing the Instrument of Surrender on September 2, 1945, officially ending World War II.

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

Manhattan Project: Trinity site test tower, New Mexico, July 1945Manhattan Project: 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 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.

Manhattan Project: Bomb assembly group leader, Trinity site, New Mexico, July 15, 1945Manhattan Project: 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 0.016 of a second after test detonation.

Manhattan Project: Aerial view of ground zero, New MexicoManhattan Project: 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), 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.

University of California Television: The Manhattan Project