Why We Fight is a series of seven propaganda films produced by the US Department of War from 1942 to 1945, during World War II. It was originally written for American soldiers to make them support the United States' involvement in the war, but US President Franklin Roosevelt ordered distribution for public viewing.

Academy Award-winning filmmaker Frank Capra, daunted but impressed and challenged by Leni Riefenstahl's 1935 propaganda film Triumph of the Will, worked in direct response. The series faced various challenges, such as convincing a noninterventionist nation to get involved in the war and to become an ally of the Soviet Union. Many entries feature Axis powers' propaganda footage from up to 20 years earlier, recontextualized to promote the Allies.

Although primarily edited by William Hornbeck, some parts were re-enacted "under War Department supervision" if no relevant footage was available. Animated segments were produced by Walt Disney Productions, and the animated maps followed a convention of depicting Axis-occupied territory in black. In 2000, the US Library of Congress deemed the films "culturally significant" and selected them for preservation in the National Film Registry.

After the Japanese attack on Pearl Harbor in December 1941 and America's subsequent joining World War II, American film director Frank Capra enlisted with the armed forces. He was already an established Hollywood director responsible for directing Academy Award-winning films such as It Happened One Night (1934) and Mr. Smith Goes to Washington (1939). He was assigned to work directly under George Marshall, the Chief of Staff of the United States Army. Marshall felt that the Signal Corps, an army branch responsible for communications, was incapable of producing "sensitive and objective troop information films". One colonel explained the importance of these future films to Capra:

You were the answer to the General's prayer. ... You see, Frank, this idea about films to explain "Why" the boys are in uniform is General Marshall's own baby, and he wants the nursery right next to his Chief of Staff's office.

During his first meeting with Marshall, Capra received his mission:

Now, Capra, I want to nail down with you a plan to make a series of documented, factual-information films—the first in our history—that will explain to our boys in the Army why we are fighting, and the principles for which we are fighting. ... You have an opportunity to contribute enormously to your country and the cause of freedom. Are you aware of that, sir?

Shortly after his meeting with General Marshall, Capra viewed Leni Riefenstahl's "terrifying motion picture", Triumph of the Will. Capra describes the film^[when?] as "the ominous prelude of Hitler's holocaust of hate. Satan couldn't have devised a more blood-chilling super-spectacle. ... Though panoplied with all the pomp and mystical trappings of a Wagnerian opera, its message was as blunt and brutal as a lead pipe: We, the Herrenvolk, are the new invincible gods!"^{[page needed]}