December 8th, 2021
One day after the surprise attack on Pearl Harbor by Japanese ships, the United States enters the fray with a declaration by Congress on December 8, 1941 against Japan. However, the official entry into World War II came on December 11, 1941. Ultimately the Second World War stretched for six years. The conflict would take more lives and destroy more land and property around the globe than any previous war.
Source: historycooperative.org/when-did-the-us-enter-ww2/