1) On December 7, 1941, Japanese forces launched a surprise attack on the U.S. naval base at Pearl Harbor in Hawaii. Over 2,400 Americans were killed and many ships and aircraft were destroyed in the attack.
2) In response, the U.S. entered World War 2 by declaring war on Japan. Germany and Italy also declared war on the U.S. a few days later.
3) The attack on Pearl Harbor was intended to neutralize the U.S. Pacific fleet so that Japan could invade territories in Southeast Asia without interference. However, it had the effect of bringing the fully industrial might of the U.S. directly into the war against Japan and its Axis allies.