This scene always gets us going.
Victor Laszlo, fed up with the Nazis singing their Nazi song in their Nazi manner, tells the band to start playing the French national anthem, "La Marseillaise," to counter it. Everyone joins in, the Germans look cranky and Rick's bar ends up getting closed down. (Though since he let the whole thing happen, he has no one to blame but himself.)
So what's the big deal? First, this is a big step forward for Rick: stopping that whole "I'm just here to serve drinks" routine and actually picking sides. But whose side is he picking? And why is this song in particular so important?
Like we mentioned, "La Marseillaise" is the French national anthem. But the Nazis largely occupied France in the early 1940s when the movie was made, and nobody played that song on German turf. In fact, it was flat-out banned: Vichy France, which technically ran Casablanca, used a song called "Maréchal Nous Voilà" as its anthem. As a result, "Le Marseillaise" became a theme of the French Underground, and of similar French Resistance forces trying to free their country from German rule.
So singing it like they do is pretty much a "stick a thumb in yer eye" gesture on behalf of the whole bar. And not only did it signal to us that Rick was starting to wake up, but it showed us how important the ideals it represented were for Victor Laszlo. The guy's being hunted. He's two steps from getting thrown into a concentration camp. And yet he still feels the need to put those Nazi bullies in their place and remind people that they don't have to put up with being the German's tools.
That's how Laszlo rolls…and why the Germans want him so badly. Guys like this cause trouble, and it usually turns out to be more than just singing a song in a quiet bar.