Kurt Julian Weill was a German composer, active from the 1920s in his native country, and in his later years in the United States. He was a leading composer for the stage who was best known for his fruitful collaborations with Bertolt Brecht. With Brecht, he developed productions such as his best-known work "The Threepenny Opera", which included the ballad "Mack the Knife". Weill held the ideal of writing music that served a socially useful purpose. He also wrote several works for the concert hall. He became a United States citizen on August 27, 1943.