I am afraid I must beg to differ from the previous reviewer. This book is (of course) very good as a military history. However, despite its title it really does not aspire to be more than that (as the notes and bibliography reveal) - and there is a great deal more than that to the Civil war. So it is far from being a definitive book about the war - and if you are looking for that you may well be disappointed.
However it is excellent on the real military history aspects. Keegan has conveyed more clearly to me the impact of geographical factors, both generally and in relation to specific battles, than anything else I have read. He is excellent on identifying and keeping track of the planning of the war on each side. He is also wonderful at evaluating the individual generals, and drawing speaking parallels with generals of other wars. Frankly, I could have used and enjoyed more detail on each of these facets, but particularly the last.
The two big problems with the book are (1) the title, which conveys a false sense of the ambit of the book - it should more properly be called "A Military History of the Civil War" or "The Geography, Battles and Generals of the Civil War" (2) the introductory chapters (and to some extent parts of the concluding chapters too) where Keegan goes outside the military history remit; these sections are frankly somewhat carelessly written and edited (so much repetition - including the same phrase twice within two paras -that I came close to throwing the book across the room) and say nothing illuminating at all. He would have been much better to drop these, cut to the chase, and give us some more of what he is really good at.