United States declared war against Japan and thus entered World War II in 1941. The declaration of war was a reaction to the December 7th bombing of Pearl Harbor. Approximately 16 million people fought in the war or served as support for the military. Over 400,000 people died in action. Almost everyone who remained in America supported the war effort through rationing, buying war bonds, sending packages overseas, and collecting metals that could be used for military purposes. World War II ended in 1945. Children could understand more about the war by reading World War II Days: Discover the Past with Exciting Projects, Games, Activities, and Recipes by David C. King and Cheryl Kirk Noll.