- December 7: The attack on Pearl Harbor by the Imperial Japanese Navy on December 7, 1941, led to the United States entering World War II on the side of the Allies.
- World War II: American troops fought in various battles and campaigns across Europe, the Pacific, and North Africa.
- Internment of Japanese Americans: In response to fears and racism following Pearl Harbor, President Franklin D. Roosevelt signed Executive Order 9066, authorizing the internment of Japanese Americans in camps throughout the Western United States.
- Manhattan Project: The United States government initiated the top-secret Manhattan Project, which aimed to develop the atomic bomb.
- Atlantic Charter: President Roosevelt and British Prime Minister Winston Churchill created the Atlantic Charter, outlining the shared goals and principles for the postwar world.
- Lend-Lease Act: The U.S. Congress approved the Lend-Lease Act, providing vital supplies and equipment to the Allied countries, particularly Great Britain, the Soviet Union, and China, in their war efforts against the Axis powers.
- Zoot Suit Riots: In Los Angeles, conflicts arose between American servicemen and young Latinos wearing "Zoot Suits," reflecting tensions and ethnic rivalries.
- Double V Campaign: African American civil rights organizations promoted a "Double V" campaign, advocating for both victory against fascism abroad and the struggle for racial equality at home.
- Rosie the Riveter: Women's participation in the war effort increased, exemplified by the iconic symbol of "Rosie the Riveter" representing the women who filled industrial and manufacturing jobs typically held by men.