Czechoslovakia's jazz roots were established by Jaroslav Ježek and Rudolf Antonín Dvorský in the 1920s and 1930s. Ježek's influence in this realm is particularly noted and by the time he immigrated to the United States in 1939, his compositions blending jazz and classical music were among the most popular music. After the invasion of Czechoslovakia by the Nazis, however, jazz was banned and it was not until 1947 when the Australian jazz pianist Graeme Bell and his Dixieland Jazz Band performed at a World Youth Festival in Prague that the jazz movement was revived.