Simply put, Aaron Copland is one of the most important composers of the twentieth century. He codified a concert music sound that was distinctly "American," and single-handedly created a generous path for composers in his native land, bringing to "the new world" what was mostly, up until that point, an imported and exclusively European art form. He lived a long and productive life, one that spanned almost the entire century, and worked not only as a composer, but as a conductor, pianist, teacher, author, concert promoter and generous friend to music.
His parents immigrated from Poland and Lithuania to New York City in their adolescence; Aaron was their fifth child, born in 1900. He was gifted at the piano, studying with Victor Wittgenstein and Clarence Adler. He took lessons in harmony and counterpoint with Goldmark, an old-fashioned teacher who was dedicated to Beethoven and Fux, and against whom Copland rebelled, as children do, becoming enamoured of Scriabin, Debussy and Ives (whom Goldmark called "dangerous"). Eventually, when he was twenty, the young composer-to-be set sail for Paris and the tutelage of Nadia Boulanger.
Under Boulanger Copland began to write his first full-fledged pieces, as well as keeping an involved notebook of musical ideas, an example of his legendary parsimonious nature, always thrifty with everything, especially notes. Using the new knowledge his teacher had imparted to him about Stravinsky, he also cultivated an interest in Jazz, mostly as seen through the eyes of the French, though he did have some knowledge of it from his time in New York. When he returned to America, it was with the score to a ballet called Grohg, along with sketches that would become the Symphonic Ode and Music for the Theatre in his satchel. He had begun to find his voice.
Copland's legendary compositional output changed it all; Americans no longer had to seek Teutonic refuge, trying to be Brahms. Works like the Third Symphony, with its Fanfare for the Common Man, Rodeo, Billy the Kid, Appalachian Spring, the Twelve Poems of Emily Dickinson and El Salon Mexico showed American composers that it was possible to create a sound that was both nationalistic and musically sophisticated. He taught generations to draw influence from what was around, whether that happened to be Gershwin, Debussy, jazz, folk-tunes or hoedowns.