U.S. Naval Base
Pearl Harbor is a Naval Base located on the island of Oahu, Hawaii, west of Honolulu. Around 18,000 U.S. Army and Navy men were stationed at Pearl Harbor. They also had a massive fleet with 8 USS Battleships, including the Arizona, California, Maryland, Nevada, Oklahoma, Tennessee, West Virginia, and Pennsylvania. They also housed over 100 warships and had airfields holding hundreds of warplanes.
USS Nevada
Hawaii from Japan
Hawaii is approximately 4,000 miles away from Japan, so the attack was unexpected. The U.S. was strengthening their naval bases in the Philippines and Guam because these were close targets for Japan. So, Pearl Harbor's naval facilities weren't well protected, and their airplanes were lined up on their airfields, giving an easy target.