A few years ago the author, Chris Brown, gave a lecture to a group of fifty eminent historians. He asked them two questions: had they ever written about war? Of course they all had, history is war. And how many had read a book about the theory of war? There was a resentful silence. This remarkable admission initiated the writing of An Introduction to War Theory. In the same vein, the pre-eminent military historian Michael Howard, having lectured young army officers at Sandhurst on the Italian Campaign of 1942, was asked a single question by an impatient captain: OK, but what were its lessons? Here are those lessons, distilled. This book is for the reader who is starting his or her journey in war theory – students, journalists, junior military professionals – and anyone with a general interest who would like to know more about how wars actually work.