When did the US join for World War 2?

The United States joined World War II on December 7, 1941, following the Japanese attack on Pearl Harbor. This surprise military strike led to a formal declaration of war against Japan the next day, December 8. Shortly after, on December 11, 1941, Germany and Italy declared war on the United States, prompting the U.S. to respond in kind. The U.S. involvement significantly shifted the dynamics of the war in favor of the Allies.