Hans Delbruck (November 11, 1848 - July 14, 1929) was a German historian. Delbruck was one of the first modern military historians, basing his method of research on the critical examination of ancient sources, the use of auxiliary disciplines, like demography and economics, to complete the analysis and the comparison between different epochs to trace the evolution of military institutions.

From the eighth century through the Middle Ages feudalism determined the nature of European warfare. Medieval Warfare begins in the time of Charlemagne, who maintained a military system of freemen and of vassals bound to him in service for lands granted in fief. These pages are crowded with recreations of famous events like the Battle of Hastings and movements like the Crusades; with the brightest flowers of knighthood, and with the mercenary grandeur of Byzantium.

Hans Delbruck shows how feudal military organization varied in different countries and why the knightly forces could not hold up against the barbarous Normans. He studies military developments in the kingdoms that rose with the collapse of the Carolingian Empire, as well as the trend toward mercenary armies. When the Swiss peasants, forming the first true infantry, defeated the Burgundian knights in the fourteenth century, the era of modern warfare had begun.