The attack was a preemptive strike by the Japanese to prevent the United States Pacific Fleet from interfering with Japan's plans to expand its empire in Southeast Asia. The Japanese hoped that by destroying the Pacific Fleet, the United States would be unable to launch a counterattack and would be forced to negotiate a peace treaty favorable to Japan.
However, the attack had the opposite effect. The United States was outraged by the attack and immediately declared war on Japan. The attack also united the American people and led to a massive mobilization of resources for the war effort.