Early in the morning of December 7, 1941, Japanese submarines and carrier-based planes attacked the U.S. Pacific fleet at Pearl Harbor. Nearby military airfields were also attacked by the Japanese planes. Eight American battleships and 13 other naval vessels were sunk or badly damaged, almost 200 American aircraft were destroyed, and approximately 3,000 naval and military personnel were killed or wounded. The attack marked the entrance of Japan into World War II on the side of Germany and Italy, and the entrance of the United States on the Allied side.
The surprise Japanese attack on Pearl Harbor brought the United States into World War II. The war had been going on in Europe since September 1939 and in the Far East since the Japanese invasion of China in 1937. Holding fast to neutrality, the United States had stayed out of both conflicts. Much aid had been provided to Great Britain, in spite of a policy of declared American neutrality, but the United States would not consider declaring war unless there was a deliberate provocation.
This provocation was provided by the Japanese attack on Pearl Harbor, Hawaii, where the American Pacific Fleet was based. The attack was planned by Admiral Yamamoto Isoroku, commander-in-chief of Japan's combined fleet. The fleet that sailed to the attack on November 26 was commanded by Admiral Nagumo Chuichi. It had six aircraft carriers, two battleships, three cruisers, 11 destroyers, and about 360 planes. The planes took off when the fleet was about 275 miles (440 kilometers) north of Hawaii. The first wave reached Pearl Harbor at 7:55 a.m. Hawaii time.
The American ships were like sitting ducks in the harbor, and since it was early Sunday morning, many were not fully manned. The battleship Arizona was destroyed, with all hands on board. The California, Nevada, and the West Virginia sank in shallow water. The Oklahoma capsized. Other ships and about 180 airplanes were destroyed or badly damaged. More than 2,300 American military personnel were killed. The next day Roosevelt described the event as a "date which will live in infamy" in a speech to Congress, which promptly declared war on Japan. A few days later Germany and Italy, bound by treaty to Japan, declared war on the United States.
Japan's intention in attacking Pearl Harbor was to disable the American fleet in order to wage a war of conquest across the eastern Pacific without opposition. It nearly worked, but two things went wrong. First, American aircraft carriers were not in port when the attack came, and carriers would prove pivotal in fighting the Pacific War. Second, the Japanese did not bomb the vast oil supply adjacent to the harbor--thus leaving a huge fuel supply for the ships and planes that did survive.See more here