This is a really good read. Keegan goes back to the stone age and works his way forward, covering the things that (in his opinion) revolutionized war in human history. It's one of those books in which I think he's probably generally right, although perhaps over-emphasizing some things and underplaying others to accord with his general thesis. In any case, it's thought-provoking whether you agree or not.
There's also a rather sad touch in his epilogue (written in 1993) in which he looks forward to a world where war is less and less an option and the United Nations and major powers work to keep things civilised. This seems perhaps further away now than it must of to Keegan then (when the collapse of the Soviet bloc was still recent). Hopefully he's right anyway, even if it happens later than he hoped.
A History of Warfare (Amazon)