At the beginning of the 20th century, the military importance of the Hawaiian Islands became clear. Oahu in particular was a key bastion in projecting America's military power in the Pacific. The island was turned into a military fortress and yet it also became the site of one of America's greatest defensive failures, the Japanese attack of December 7, 1941. By the end of World War II, the harbor itself was the most heavily defended in the world, and the island had earned the sobriquet “Fortress Oahu”. This title documents the development of the coastal, air and land defense systems that served to protect Pearl Harbor and Honolulu from 1907 to 1950, and seeks to understand why these failed at a critical point.