The U.S.-soldiers called the locals Việt Nam Cộng Sản (Vetnamese Communist) and shortened that down to Viet Cong. In the NATO-alphabet this was Victor, Charlie and this way the enemy was reduced to Charlie.
It is one mean of dehumanizing in an effort of war: reducing the opponent to a faceless mass without individuality and demonizing them through stereotypes. In this sense Warldwar Z is just a continuation of this thought …