On June 24, 1944, U.S. Navy warplanes patrolling the Atlantic attacked and sank a Japanese submarine, the I-52. It was an event of enormous strategic importance. The I-52's mission was to return to Japan with the ingredients of a radiological bomb. Purchased from the Germans, this doomsday weapon-classified secret by the U.S. government for years after the war-could have contaminated vast areas of America for decades, killing millions slowly and painfully...