Sun Tzu's Art of War
Must be in Middle Ages.
In the unsettled times following the collapse of Charlemagne's Frankish empire, a new social and political system arose in Europe called feudalism. Derived from the concept of monarchy, feudalism was a hierarchical system in which each descending stratum owed allegiance to those above. The king, whose authority was second only to God, granted tracts of land, or fiefs, to his barons. The barons might then grant smaller estates to knights, who swore to protect and serve the baron. The knights, in turn, might then grant fractions of their fiefs to subordinates who would serve the knights. At the bottom of the feudal pyramid were the serfs, who could not own land, and acted essentially as slave laborers who worked the land for their masters. Feudalism began to decline in the 13th century, when standing armies and mercenaries began to fulfill the protective role once served by the vassals and knights.