Why Pearl Harbor became the Pacific Fleet's vulnerable fulcrum

Pearl Harbor sat near important Pacific routes and served as a base for American power. The Pacific Fleet had previously been based at San Diego before moving forward to Hawaii. Detailed planning for the December 1941 operation probably began in January, while the underlying concept could have been studied for years. Taranto offers a comparison: detailed operational planning began around April 1940, while a British carrier-strike concept had existed since about 1925. The distinction between a standing concept and an authorized operation makes an earlier Japanese planning history plausible, without establishing its exact beginning.