On December 11, 1941, the United States entered World War II. Mobilization began on December 8, 1941, when the United States declared war on Japan, one day after the Pearl Harbor assaults.

The Japanese attack on Pearl Harbor prompted the United States of America to enter World War II. However, prior to the attack on Pearl Harbor, the United States took an indirect role in the war by selling munitions with Allied countries such as Britain. President Franklin D. Roosevelt perceived Nazi Germany as a threat to the United States and believed that arming the Allied democracies in Europe was important.
The United States began the war neutral, but ideologically supported the Allies. Although Congress did not initially approve military force against the Axis, the passage of the Lend-Lease Act was the first step toward entering World War II. While transporting Lend-Lease supplies to the Allies, the USS Greer was assaulted by Germans, prompting President Roosevelt to declare a shoot-on-sight policy.
The real, direct military engagement began only after the attack on Pearl Harbor. The Japanese provocation prompted Congress to vote virtually overwhelmingly to declare war on Japan. Adolf Hitler, the leader of Nazi Germany, declared war on the United States shortly after the attack on Pearl Harbor. These events resulted in America's direct military involvement in both the European and Pacific theaters, finally leading to the Axis countries' demise.