As the title "Art of War" indicates, this isn't a history of campaigns and battles per se but the evolution of military strategy, tactics and weapons in this period. The overarching theme is the ascendancy and eclipse of the armored horseman which began as the need to find have a force of mobile, professional soldiers to deal with the Vikings and the Magyars. This first phase culminates in the battle of Hastings where a mounted army defeated an infantry army. It goes on to cover the decline of the armored horseman. This showed up in the victories of the English longbow and infantry armies and in pike formations of the Swiss. Several key battles are mentioned as examples of the various types of battle (cavalry vs. infantry, cavalry and infantry vs. cavalry, etc.). Of interest is the lack of strategic sense in Western Europe, Prince Edward's Evesham campaign being a rare exception. A close look is also taken at the Byzantine system which was marked by a pragmatic and flexible adaptation to the different tactical systems of their potential enemies. Discussion also covers the armies of the Vikings, Saracens and Magyars, et al. There are also chapters in each chronological period covering developments in arms and armor and in fortification and siege craft. I have two criticisms of the work. The first is his chapter on the Mongols. Oman writes off the Mongols' victories in Europe as being due to the disunity of the Europeans and Mongolian numerical superiority. In fact, Genghis Khan organized a first-rate military system marked by the use of maneuver and exercised by disciplined troops that was later led by capable heirs (see the discussion the Mongols in Dupuy and Dupuy's Encyclopedia of Military History). At Liegnitz, contrary to being outnumbered fivefold, Henry the Pious had numerical parity with the Mongol force. King Bela's army at the Sajo River was about equal to Subotai's force. I'm also a little dubious about his assertion that gunpowder wasn't of Chinese origin. None of his examples arguing that Chinese weapons were actually incendiaries address the claim that the Chinese used gunpowder in firecrackers (thus, they may not have been the first to find a military application for gunpowder). Those two areas, however, are only a small portion of the work which is well-written and well worth the time of anyone interested in the subject.