Casablanca was, at the time it was made, "nothing special." It was just another film in a string of many released during World War II. It is based on a play named "Everybody Comes to
Rick's" written by a gentleman who, in turn, based the play on a real trip to the city of
Casablanca during that period.
Later, however, it became a cult classic (some say at Harvard) which caused it to enjoy a renaissance and re-examination by popular culture - and it took off.
Some interesting trivia about the movie; for one thing, the entire thing was filmed in a back lot in Hollywood. Many tons of sand were trucked in and laid down to create the illusion of a desert town.
The war meant that getting real airplanes was impossible. The DC-3 pictured at the end, which Ilsa and Victor are to board, is fake; it's a perspective trick, too, being a 1/2-scale model. The ground crew attending it is composed of midgets so as not to break the illusion.