It took the US army two full years after the attack on Pearl Harbour to break the radio codes of the Japanese Imperial Army. But by 1944, the US was decoding more than 20,000 messages a month filled with information about enemy movements, strategy, fortifications, troop strengths and supply convoys. In MacArthur's ULTRA, historian Edward Drea recounts the story behind the army's painstaking operation and its dramatic breakthrough. He demonstrates how ULTRA (intelligence from decrypted Japanese radio communications) shaped MacArthur's operations in New Guinea and the Philippines. By correlating the existing intelligence with MacArthur's operational decisions, Drea shows how MacArthur used - and misused - intelligence information. He also clarifies the role of ULTRA in Truman's decision to drop atomic bombs on Japan in 1945, and examines the role of ULTRA on the outcome of World War II.