This chapter provides a discussion of how scientists and non-scientists view war and whether it is a natural phenomenon. It begins with offering a working definition of war and identifying various types of warfare: interstate (international) v. intrastate (civil), limited v. total, and regular v. irregular. The chapter explores such subjects as prehistoric warfare, warfare and state-building, the early modern military revolution, insurgency and counterinsurgency, and the twenty-first-century warfare. It reviews literature on the causes of war identified at the level of individuals, states, and the international system. Scholars diverge widely in their explanations of the causes of war. Sociologists argue that most of the time a single powerful state dominates the system and imposes its rules of the game, including trade, diplomacy, communications, use of force, and so on. This hegemonic transition was associated with a period of political turbulence and hegemonic wars.