
from wikipedia:
The attack on Pearl Harbor[nb 4] was a surprise military strike by the Imperial Japanese Navy against the United States naval base at Pearl Harbor, in the United States Territory of Hawaii, on the morning of December 7, 1941. The attack led to the United States' entry into World War II.
Japan intended the attack as a preventive action to keep the U.S. Pacific Fleet from interfering with military actions the Empire of Japan planned in Southeast Asia against overseas territories of the United Kingdom, the Netherlands, and the United States. There were near-simultaneous Japanese attacks on the U.S.-held Philippines, Guam and Wake Island and on the British Empire in Malaya, Singapore, and Hong Kong.[13] The attacks - from troop landings at Kota Bharu, Malaya, to the air attacks ranging geographically from Hong Kong to Pearl Harbor - took place over seven hours.
The attack came as a profound shock to the American people and led directly to the American entry into World War II in both the Pacific and European theaters. The following day, December 8, the United States declared war on Japan.[18] Domestic support for non-interventionism, which had been fading since the German attack on France in 1940,[19] disappeared. Clandestine support of the United Kingdom (e.g., the Neutrality Patrol) was replaced by active alliance. Subsequent operations by the U.S. prompted Germany and Italy to declare war on the U.S. on December 11, which was reciprocated by the U.S. the same day.