War is a social activity as old as civilisation itself. To refer to so unpleasant an activity as being ‘social’ is something of a misnomer, but nearly all societies have known war (Keegan 1994; Keeley 1996). Raymond Aron (1905-1983) discussed the biological and psychological roots of war and concluded ‘that man is by nature dangerous to man’ (Aron 1981: 344). Most famously, war was defined by the Prussian philosopher of war, Carl von Clausewitz (1780-1831), as ‘a continuation of political intercourse, carried on with other means’ (Clausewitz 1976: 87). This chapter will concern itself with certain aspects of the evolution of the conduct and study of war. It will deal with inter-state and sub-state conflict (the latter encompassing civil wars). It will also highlight a number of major debates about the history of warfare and outline a preliminary approach for those wishing to study it.