This extended meditation on the clash of armies takes in all parts of the world where societies have come into conflict - from the closed and insulated 'total wars' of the Easter Islanders to the all-in world wars of our time. From the start Clauswitz is robustly contradicted: 'War is not the continuation of policy by other means,' says Keegan, who prefers the crisper verdict of Sherman: 'Glory is all moonshine. War is hell.' The argument is underpinned by hope: war is not inevitable, but a habit. But it's a habit we've failed to kick for 4,000 years and - modern weapons being what they are - the time available for us to kick it is running out.