They deliberately moved their fleet closer to Japan in order to instigate them into attacking first:

"The primary naval force that eventually called Pearl Harbor was moved from the U.S. West Coast. In May 1940, President Franklin D. Roosevelt ordered the U.S. Pacific Fleet to leave its home port in San Pedro, California, and take up a permanent, forward-positioned base at Pearl Harbor as a deterrent to Japanese expansionism. The Pacific Fleet's headquarters was officially transferred from San Diego to Pearl Harbor in 1939, which accelerated the base's development before the fleet's main body arrived in 1940."