On August 6, 1945, 30-year-old U.S. Air Force pilot Col. Paul W. Tibbets Jr. took to the sky in the Enola Gay, his Boeing B-29 Superfortress heavy bomber. His destination, the Japanese city of Hiroshima, was not an especially notable target. His payload, however, a single bomb nicknamed “Little Boy,” would change the course of history. True watershed moments in history are rare — the agricultural revolution is one such example, as was the Battle of Salamis, the advent of Jesus Christ, and the fall of Western Rome. Yet in the last 1,500 years, no two distinct epochs of time...