His Royal Highness (1918)

Football star Jack Christie accompanies his college roommate Victor Borden to France, where the latter becomes attached to gangster's daughter Lisette. He is injured trying to save her from a forced marriage, and while he is confined in a hospital, he reveals to Jack that he is actually Prince of Wallarya, a small country in the Balkans. Because Prince Ferdinand, who wishes to seize the throne, has convinced the King of Terresta to declare war on Wallarya, Jack agrees to take Victor's place temporarily at the head of the army. Posing as the prince, Jack foils Ferdinand's attempts to assassinate him and then prevents war by agreeing to marry the king of Terresta's daughter Diana. Victor wires Jack that he plans to settle permanently in America with Lisette, leaving the football hero to rule Wallarya with his bride.