My argument is: Plato and Aristotle have a pacifist vein. Nevertheless, their best cities are both war-like communities and both defend slavery and wars against barbarians in order to acquire slaves. These apparent contradictions are examined. Plato’s and Aristotle’s views on war and peace are considered to be similar. The human soul and its appetites drive history and cause war. Accordingly, a well-ordered soul and city are the premise of curbing man’s political urge to conquer and to go to war. This insight plays a crucial role in Plato’s and Aristotle’s psychology and their constitutional thought.