The USA entered World War II in response to the Japanese bombing of the American naval base at Pearl Harbor, Hawaii. Prior to the attack, the country had maintained an isolationist policy, although government leaders considered involvement inevitable and had been providing the Allies with arms and other supplies. The US had long-standing friendly relations with Britain and the Japanese assault tipped the country against the Axis powers.
Until 1941, the US kept out of the war. In general, Americans considered it a European affair and preferred to follow the traditional American policy of isolationism. However, the country was sympathetic to the Allied cause. The US had fought against Germany during WWI. Communists, leftists and Jews who heard about the oppression occurring in Germany pressed and lobbied the government to intervene. Many people believed that the totalitarian tendencies of European fascism would eventually threaten the US. In addition, the US shared a common heritage and friendly relations with Britain, whose very existence was threatened by the Germans.
For these reasons, the US sided with the Allies from the beginning and furnished them with much needed war supplies. The Roosevelt administration anticipated involvement and prepared the nation's arms manufacturing. When the Japanese bombed Pearl Harbor, it was the final component necessary to remove any doubt about going to war.