Complex sentence: Burnham was a radical activist in the 1930s and an important factional leader of the American Trotskyist movement, while in later years, he turned to conservatism and served as a public intellectual of the conservative movement.
Split into simpler sentences: Burnham was a radical activist in the 1930s and an important factional leader of the American Trotskyist movement. He later turned to conservatism and served as a public intellectual of the conservative movement.