Casablanca was filmed as WWII raged and the 🇨🇵 singers in the Rick's bar scene were actually French refugees. Their emotions were very real as they belted out La Marseillaise to drown out the German officers singing Deutschland Uber Alles.
Small correction: the Nazi officers are singing Watch on the Rhine, not Deutschland über Alles. The former is specifically anti-French:
en.m.wikipedia.org/wiki/Die_Wac...