It is a historical commonplace that major revolutions in military techniąues have usually been attended with widely ramifying conseąuences. The coming of the mounted warrior, and of the sword, in the middle óFtKe second millennium b g ; the triumph of the heavy cayalryman, Consolidated by the adoption of the stirrup, in the sixth century of the Christian era; the scientific reyolution in warfare in our own day - these are all recognized as major turning-points in the history of mankind. The period in the history of the art of war with which I shall try to deal in this lecture may seem from this point of view to be of inferior importance. But it brought changes which may not improperly be called a military reyolution ^and that reyolution, when it was accomplished, exercised a profound influence upon the futurę course of European history. It stands like a great divide separat-ing mediaeval society from the modern world^j Yet it is a reyolution which has been curiously neglected by historians. The experts in military history have mostly been content to describe what happened, without being overmuch concerned to tracę out broader effects; while social historians have not been very apt to believe that the new fashions in tactics, or improvements in weapon-design, were likely to prove of much significance for their purposes. Some few sociologists, indeed, have realized the importance of the problem; but historians tend to find their expositions a trifle x>paque, and their conclusions sometimes insecurely groundea.(Yet it remains true that purely military developments, of a strictly technical kind, did exert a lasting