The postcard contained only two words: “Hurry up.”

John Archibald Wheeler, a 33-year-old physicist, was in Hanford, Wash., working on the nuclear reactor that was feeding plutonium to Los Alamos, when he received the postcard from his younger brother, Joe. It was late summer, 1944. Joe was fighting on the front lines of World War II in Italy. He had a good idea what his older brother was up to. He knew that five years earlier, Wheeler had sat down with Danish scientist Niels Bohr and worked out the physics of nuclear fission, showing that unstable isotopes of elements like uranium or soon-to-be-discovered plutonium would, when bombarded with neutrons, split down the seams, releasing unimaginable stores of atomic energy. Enough to flatten a city. Enough to end a war.

After the postcard’s arrival, Wheeler worked as quickly as he could, and the Manhattan Project completed its construction of the atomic bomb the following summer. Over the Jornada del Muerto Desert in New Mexico, physicists detonated the first nuclear explosion in human history, turning 1,000 feet of desert sand to glass. J. Robert Oppenheimer, the project’s director, watched from the safety of a base camp 10 miles away and silently quoted Hindu scripture from the Bhagavad Gita: “Now I am become Death, the destroyer of worlds.” In Hanford, Wheeler was thinking something different: I hope I’m not too late. He didn’t know that on a hillside near Florence, lying in a foxhole, Joe was already dead.

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