When World War II started in 1939, Horowitz decided to live in the U. S. He became an American citizen in 1944. Horowitz was very proud of being an American, and decided to make a piano version of Sousa's The Stars and Stripes Forever. He played many concerts to raise money for the war effort, and he asked people to call him "the American pianist." Horowitz began playing music written by an American composer, Samuel Barber, and gave the world’s first performances of the Barber's Piano Sonata and Excursions.