In 1943, with the future of the free world hanging in the balance, the United States military began to plan one of the most dramatic secret operations of World War II. In April of that year, multiple U.S. Naval Intelligence stations intercepted the itinerary of Japanese Admiral Isoroku Yamamoto, the Commander-in-Chief of the Japanese Combined Fleet and the mastermind of Pearl Harbor. Yamamoto, Harvard-educated and a close confidant of Hirohito, was a brilliant military tactician and, symbolically, perhaps the most important symbol of Japanese military might. On April 18th, the U.S. discovered, he would be visiting Rabaul in the South Pacific to boost the morale of Japanese troops. Afterward, he would fly to the Japanese airfield at Balalale, 400 miles to the southeast. Thus was set in motion Operation Vengeance, one of the most daring, and tactically difficult, operations of World War II.