How did Casablanca deliver a political message?

Casablanca is an exploration of the universal themes of love and sacrifice, but when the film was released in 1942, audiences viewed it as a political allegory about World War II. The film is set in December 1941, the month in which the Japanese attacked Pearl Harbor. That attack changed the course of American history, awakening the nation from political neutrality and thrusting it into the midst of World War II. Casablanca tells the story of a similar, though much smaller, awakening. At the beginning of the film, Rick is a cynical bar owner in the Moroccan city of Casablanca who drinks only by himself and doesn't care about politics. By the end of the film, he has become a self-sacrificing idealist, committed to the anti-Nazi war effort. The event that prompts this change in Rick is the appearance of Ilsa, his old flame, in Casablanca. Ilsa's arrival is unexpected and devastating, and it hits Rick just as hard as the Japanese sneak attack on Pearl Harbor hit America. Once Rick overcomes the initial pain, his moral sense is reignited. He doesn't get to live happily ever after with Ilsa, but he accepts the necessity of his sacrifice and the heartbreak that accompanies it. I