When Did The US Join World War II?

The United States of American joined in the World War II after 7th of December, 1941; the day on which it's Pearl Harbor was attacked. Prior to this America had stayed away from the war and wasn't a part of either the Allied or the Axis powers. However, when Japan destroyed almost the entire American navy with a surprise bombing of Pearl Harbor which was of its primary naval bases, the U.S joined in fighting along side France and Britain. In the January of 1942, the United Nations Declaration listed out the 26 other countries that would be a part of the Allied Powers. Incidentally, this declaration also became the founding declaration for the United Nations. In the latter half of World War II, the United States along with the Soviet Union and the United Kingdom became the 'Big 3'; the crux of the Allies Powers who eventually defeated their Axis rivals.