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Weill wrote some of the most recognizable music of the twentieth century as Bertolt Brecht's collaborator on The Threepenny Opera, whose ballad "Mack the Knife" long outgrew its stage origins. He believed music should serve a social purpose, and pursued that conviction from Weimar Germany's cabarets to the Broadway of Lady in the Dark and Street Scene after he fled the Nazis in 1933. Few composers moved as fluidly between European high modernism and American popular theater.