"Play La Marseillaise! Play it!"
A Germans stand around Sam's piano as they sing "Wacht am Rhein", a German patriotic song about conflicts with France. Laszlo takes command of Rick's band and has them play "La Marseillaise", the French National Anthem. The counterpoint of the two is a stunning contrast and bit by bit the crowd gathered in the bar joins in drowning out the Germans.
One of the most moving scenes in all of film and especially if you realize that a number of the cast you are watching actually have fled Europe from Nazi occupation and are refugees in America at the time this was filmed. I've seen this movie at least fifty times and my eyes are swelling even as I write this.
I believe this clip is "fair use" in discussing this subject.