Prelude to War is the first film of Frank Capra's Why We Fight film series commissioned by the Office of War Information (OWI) and George C. Marshall. It was made to educate American troops of the necessity of combating the Axis powers during World War II based on the idea that those in the service would fight more willingly and ably if they knew the background and the reason for their participation in the war. The film was later released to the general American public as a rallying cry for support of the war.

In an edit added to the film before public release, a comment by US Secretary of War Henry L. Stimson, is quoted to create trust with the audience: "the purpose of these films is to give factual information as to the causes, the events leading up to our entry into the war and the principles for which we are fighting."

The purpose of the OWI was to use mass communication to appeal to both serving soldiers and later, the American population on the reasons for the war; and so it released Prelude to War to further those goals. The film starts by an explanation of how Americans were brought into the war through heroic motives to protect countries unable to protect themselves. The film expresses the view that America had this duty to righteousness and Christian values throughout history, according to the values and the beliefs set forth by the Founding Fathers.

The film makes use of compare and contrast methods throughout the film to transmit its heroic message. The primary images used within the film to portray the opponents of America are introduced by Henry Wallace as the "free world," a brightly illuminated planet of the Allies, and a "slave world," a planet deep in shadow of the Axis powers. These maps were animated by Walt Disney Productions.

The film examines the differences between the United States and the fascist Axis states of Benito Mussolini, Adolf Hitler and Hirohito by portraying the latter countries working together as gangsters to conquer the world. That is done by footage from Nazi's Triumph des Willens (Triumph of the Will) but with different narration designed to support the Allied cause.

It is emphasized that after the Nazis smashed the opposing political parties and labor unions, they turned their attention to their last remaining obstacle, the church. In one scene, a stained glass window is shattered by several bricks to reveal a "Heil Hitler!" poster behind. To emphasize that depiction of Hitler as an Antichrist figure, a class of German schoolchildren is shown singing to the song Frederick Rex:

Adolf Hitler is our Saviour, our hero
He is the noblest being in the whole wide world.
For Hitler we live,
For Hitler we die.
Our Hitler is our Lord
Who rules a brave new world.

In the two decades following World War I and the failure of the League of Nations, a spirit of isolationism becomes prevalent throughout the United States and persists until the attack on Pearl Harbor. That presented a major obstacle in garnering support for the war, as Americans did not feel a sense of unity with other nations of the world, most notably, Europe. In an effort to dispel that isolationist way of thinking, Prelude to War was produced to raise the enthusiasm and interest of US troops for the international struggle.