Dressed to kill: A Union infantryman (Getty Images)

John Keegan is the leading military historian of the English-speaking world. He is so for five main reasons. First, his knowledge of military history is profound, encyclopaedic and ranges over an immense span of time, from antiquity to the current campaigns in the Middle East, where he has served as unofficial adviser to the White House. Second, he has the necessary ability to combine a broad understanding of wars in their overall strategic aspects, with a detailed grasp of tactics, weapons, terrain and professional soldiery. Third, he has a good nose for politics and thus is able to understand how politics and military matters interact and influence each other. Fourth, he has made a special study of the qualities that make up a great commander, and is an excellent judge of them; this gives him a particular skill in showing how individuals of military genius can determine the outcome of wars. Finally and perhaps most important, he writes with vivacity and often with notable power, describing the course of conflict with admirable clarity and bringing home to the reader the excitement and pathos, the horrible routines and the unexpected turns of events which characterise warfare, and the human spirit which gives it nobility amid all the squalor and savagery of professional killing. 

He has now turned his hand to writing a comprehensive military history of the American Civil War, a giant subject at which he has hitherto only glanced. I say a giant subject because it was a long and bitterly contested war, very evenly matched for most of its duration. The casualties were appalling. It is the central event in America's history as an independent nation and still casts long shadows. As any historian who writes about it discovers, a huge number of American males take an immense interest in the subject, and know a vast amount about it, especially in the details of its forces, commanders and, above all, its battles.

The battles are the essence of the American Civil War and give it its unique horror and interest. As Keegan writes: "By common computation, about 10,000 battles, large and small, were fought in the United States between 1861 and 1865. This enormous number of battles, seven for every day the war lasted, provides the principal key to the nature of the war." By comparison, the English Civil War musters barely a dozen, and even if we include sieges, not much over a score — one reason why the number of those killed was so small, compared to the half a million dead in the American conflict. 

It was an ideological war, in which not just the politicians, and the generals on each side, were impelled to fight by profound convictions, but the common soldiers too. It is hard to think of a war in which the rank-and-file were so strongly motivated. That is why there were so many battles and why soldiers were prepared to fight so often and so fiercely. In effect, as Keegan argues, the only way either side could win was to destroy its opponents' armies. The soldiers on both sides wanted passionately to win. The Northerners believed that the United States was the greatest nation on earth, which provided for them and their families a standard of living, and a freedom in things great and small, unobtainable anywhere else on earth. All that depended on the Union, and Lincoln persuaded them — greatly assisted by the stupidities of the Southern politicians — that the preservation of the Union was the object of the war. The Southerners were "rebs", combining to form a force of destruction which would end the United States and so their way of life.