Aaron Copland was a great pioneering force in American music, and is in fact one of the most honoured cultural figures in US history - he basically "_is_" American music!
Early on, he was influenced by Stravinskian neo-classicism, popular music, and the developing European nationalist composing traditions; And upon his return to the USA, his modernism and use of jazz gave him a reputation as an 'enfant terrible'. But in the 1930s and 40s he broke away from European music and established what is now a characteristically American vernacular style, employing simple harmonies, folk melodies, and lucid orchestration. He won great public appeal through such pieces as _Fanfare for the Common Man_ (which has since been routinely used for all events of human haughtiness, not least as a Star Trek theme tune!), _Old American Songs_, and his Pulitzer Prize-winning ballet _Appalachian Spring_.
As a conductor and an educator he worked tirelessly to promote other composers, at Harvard, Tanglewood, on radio and television.