In 1928, he wrote the music for a play by Brecht which was to become world-famous. It was called "Dreigroschenoper" ("The Threepenny Opera"). It was based on an opera called "The Beggar's Opera" which had been written in 1728 by John Gay. The "Threepenny Opera" includes a song which became Weill's most famous song, "Mack the Knife". Weill stopped working with Brecht in 1930 because they had political disagreements. Brecht was becoming more and more interested in Communism, but Weill did not want to be a “Communist composer”.