The Art of War has been very simply defined as ‘the art which enables any commander to worst the forces opposed to him.’ It is therefore conversant with an enormous variety of subjects: Strategy and Tactics are but two of the more important of its branches. Besides dealing with discipline, organization, and armament, it is bound to investigate every means which can be adapted to increase the physical or moral efficiency of an army. The author who opened his work with a dissertation on ‘the age which is preferable in a generalissimo,’ or ‘the average height which the infantry soldier should attain[1],’ was dealing with the Art of War, no less than he who confined himself to purely tactical speculations. The complicated nature of the subject being taken into consideration, it is evident that a complete sketch, of the social and political history of any period would be necessary to account fully for the state of the ‘Art of War’ at the time. That art has existed, in a rudimentary form, ever since the day on which two bodies of men first met in anger to settle a dispute by the arbitrament of force. At some epochs, however, military and social history have been far more closely bound up than at others. In the present century wars are but episodes in a people’s existence: there have, however, been times when the whole national organization was founded on the supposition of a normal state of strife. In such cases the history of the race and of its ‘art of war’ are one and the same. To detail the constitution of Sparta, or of Ancient Germany, is to give little more than a list of military institutions. Conversely, to speak of the characteristics of their military science involves the mention of many of their political institutions. At no time was this interpenetration more complete than in the age which forms the central part of our period. Feudalism, in its origin and development, had a military as well as a social side, and its decline is by no means unaffected by military considerations. There is a point of view from which its history could be described as ‘the rise, supremacy, and decline of heavy cavalry as the chief power in war.’ To a certain extent the tracing out of this thesis will form the subject of our researches. It is here that we find the thread which links the history of the military art in the middle ages into a connected whole. Between Adrianople, the first, and Marignano, the last, of the triumphs of the mediæval horseman, lie the chapters in the scientific history of war which we are about to investigate. I. THE TRANSITION FROM ROMAN TO MEDIÆVAL FORMS IN WAR. A.D. 378-582. [From the battle of Adrianople to the Accession of Maurice.]