When did the U.S. join the fight against Hitler?

The United States declared war on Japan on December 8, 1941, the day after the Japanese attacked the United States. Germany & Italy declared war on the United States on December 11, 1941. The United States declared war on Germany & Italy later that day. The United States had been helping the Allies since 1940 in several ways short of war.

Stalin declared war on Imperial Japan about one week before Japan surrendered.