Complex sentence: Copland's compositions in the early 1920s reflected the prevailing modernist attitude among intellectuals: that they were a small vanguard leading the way for the masses, who would only come to appreciate their efforts over time.
Split into simpler sentences: Copland's compositions in the early 1920s reflected the modernist attitude that prevailed among intellectuals. These individuals thought of themselves as a small vanguard for the masses, who would only come to appreciate their efforts over time.