The United States of America entered World War II due to the Japanese attack on Pearl Harbor. However, before the attack of Pearl Harbor, the United States played an indirect role in the war by trading arms with Allied countries, such as Britain. President Franklin D. Roosevelt saw Nazi Germany as a threat to the United States and believed it necessary to arm the Allied democracies in Europe.
The United States was neutral at the beginning of the war but ideologically sided with the Allies. The passing of the Lend-Lease Act by Congress was the first step toward entering World War II, although Congress did not initially authorize military force against the Axis. The USS Greer was attacked by Germans while escorting Lend-Lease supplies to the Allies, which prompted President Roosevelt to issue a shoot-on-sight policy.
Real, direct military involvement didn't begin until the attack on Pearl Harbor. The provocation by the Japanese caused Congress to vote nearly unanimously to declare war on Japan. Shortly after the attack on Pearl Harbor, the leader of Nazi Germany, Adolf Hitler, declared war on the United States. These events led to America's direct military involvement in both the European and Pacific theatres, and eventually led to the downfall of the Axis powers.