Complex sentence: Mendenhall focused on gauging the sentiment of urban-based Vietnamese and concluded that Diệm's policies increased the possibility of religious civil war, and were causing the South Vietnamese to believe that life under the Viet Cong would improve the quality of their lives.
Split into simpler sentences: Mendenhall focused on gauging the sentiment of urban-based Vietnamese and concluded that Diệm's policies increased the possibility of religious civil war. Mendenhall said that Diệm's policies were causing the South Vietnamese to believe that life under the Viet Cong would improve the quality of their lives.