Aaron Copland was the pioneer of American music -- he showed the world how to write classical music in an American way. He was born in 1900, when Americans were rarely recognised as composers in the music world. So Copland went to Europe for serious study, and, in the 1920s, wrote pieces with the flavour of jazz. European classical composers were also influenced by jazz at this time, as they were searching for new ways to bring their music into the 20th century.
George Gershwin!