Simplify the following text for a Grade 9 native English-speaking person.
Original: During World War II , Horowitz championed contemporary Russian music , giving the American premieres of Prokofiev 's Piano Sonatas Nos. 6 , 7 and 8 ( the so-called `` War Sonatas '' ) and Kabalevsky 's Piano Sonatas Nos. 2 and 3 .
Simplified: During World War II , Horowitz also played a lot of Russian music , giving the first performances in America of Prokofiev 's Piano Sonatas Nos. 6 , 7 and 8 ( which were called `` War Sonatas '' ) and Kabalevsky 's Piano Sonatas Nos. 2 and 3 .