The history of war predates history itself. It started when some member of the homo sapiens club broke off a conveniently-sized branch and clubbed the chap in the next tree with it. In the millennia since, every culture that has survived to the 21st century has done so by learning how to wage war. And yet, by John Keegan's estimate, around 70 per cent of the world's land area has never seen organised fighting simply because the terrain or the climate is inimical to the movement of large numbers of people.