Complex sentence: During the American Civil War, Indian Territory occupied most land of the what is now the U.S. state of Oklahoma and served as an unorganized region set aside for Native American tribes of the Southeastern United States after being removed from their lands.
Split into simpler sentences: During the American Civil War, Indian Territory occupied most of what is now the U.S. state of Oklahoma and served as an unorganized region. The area set aside for Native American tribes of the Southeastern United States who had been removed from their lands.