Complex sentence: They, unlike the Amish and the Mennonites who were considered sectarians did not wear plain clothing, nor did they refuse to fight in wars or do many of the things commonly associated with the "Pennsylvania Dutch" in mainstream culture.
Split into simpler sentences: They, unlike the Amish and the Mennonites who were considered sectarians. They did not wear plain clothing, nor did they refuse to fight in wars or do many of the things commonly associated with the "Pennsylvania Dutch" in mainstream culture.