The United States entered World War II in December 1941 , immediately after the Japanese attack on Pearl Harbor and the subsequent declarations of war with Japan, Germany, and Italy.

Key dates in December 1941

  • December 7, 1941: Japan launched a surprise attack on the U.S. Pacific Fleet at Pearl Harbor, Hawaii, causing heavy losses in ships, aircraft, and personnel.
  • December 8, 1941: The United States formally declared war on Japan after President Franklin D. Roosevelt’s “Day of Infamy” speech to Congress.
  • December 11, 1941: Germany and Italy declared war on the United States, and the U.S. responded with its own declarations, bringing America fully into World War II in both the Pacific and European theaters.

Simple answer version

If you need it in one line: the U.S. entered World War II in December 1941 , starting with a declaration of war on Japan on December 8, 1941 , and then war with Germany and Italy on December 11, 1941.