One of the best kept secrets of the war involved the Japanese balloon bomb offensive. Prompted by the Doolittle Raid on Tokyo in April 1942, the Japanese developed the balloon bombs as a means of direct reprisal against the U.S. mainland. The balloons, made of paper or rubberized silk, carried anti-personnel and incendiary bombs. The first operational launches took place on Nov. 3, 1944, and two days later a U.S. Navy patrol boat spotted a balloon floating on the water off the coast of California. Japan launched some 9,000 balloons during a five-month period, to be carried by high altitude...