Peconic, Long Island, New York  August 2, 1939

On this date in 1939, one month before the outbreak of World War II in Europe, distin­guished German-born profes­sor, 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 theo­ret­ical physi­cist, J. Robert Oppen­heimer. Over­seeing the nearly $2 billion project (over $22 billion in today’s dollars) 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

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

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

Manhattan Project: Aerial view of ground zero, New Mexico Manhattan 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 dark suit and light-colored hat), Brig. Gen. Leslie Groves (in khaki uni­form 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