If you only ever read one book on military history, make it this one. For where most other books of military history deal with the how, where and what of men at arms, Keegan’s magnificent book examines the ‘why’. Why is warfare perhaps mankind’s oldest occupation? In tackling this question, Keegan goes deeper even than Clausewitz, who memorably set out to demonstrate that warfare is politics by other means, for war is older than politics.

War, Keegan demonstrates, is one of the ineluctable facets of human culture and, as such, is shaped by that culture as well as shaping it. So what we might call ‘styles’ of warfare differ considerably among different cultures. When cultures were widely separated by the difficulties of travel, these different war styles operated independently within a wider cultural grouping. Thus, the Indians of the American plains counted coup – touching an enemy – rather than necessarily killing them (although this existed alongside a martial culture’s contempt for physical pain, which meant that captives could be mercilessly tortured and, crucially, the captive accepted the torture as a chance to demonstrate his own courage).

What’s changed about warfare is that it is becoming homogenised. Three millennia ago, when the tiny Greek city states fought savagely among themselves, their citizen armies could not long stay from their fields. So there grew up among them a new way of warfare: the set-piece battle, where one side was annihilated and the victors took the spoils. Among most other cultures, battles were either more formalised or less decisive: better to retreat with few losses than risk everything.

But the success of Alexander and his armies, who brought this all-or-nothing attitude to battle to Persia and, with it, destroyed the Persian Empire, brought about the gradual spread of the Greek way of war so that now it is played through most of the world, with the only other model the asymetric warfare of insurgency.

Read Keegan’s book to understand how war and culture are inextricably intertwined, each affecting the other in their lethal dance through the centuries.