Strategy and tactics
De re militari
si vis pacem, para bellum
If you want peace, prepare for war
Vegetius, De re militari, preface to book 3.
Publius Flavius Vegetius Renatus wrote De re militari (Concerning Military Matters) possibly in the late 4th century. Described by historian Walter Goffart as "the bible of warfare throughout the Middle Ages", De re militari was widely distributed through the Latin West. While Western Europe relied on a single text for the basis of its military knowledge, the Byzantine Empire in Southeastern Europe had a succession of military writers. Though Vegetius had no military experience and De re militari was derived from the works of Cato and Frontinus, his books were the standard for military discourse in Western Europe from their production until the 16th century.
De re militari was divided into five books: who should be a soldier and the skills they needed to learn, the composition and structure of an army, field tactics, how to conduct and withstand sieges, and the role of the navy. According to Vegetius, infantry was the most important element of an army because it was cheap compared to cavalry and could be deployed on any terrain. One of the tenets he put forward was that a general should only engage in battle when he was sure of victory or had no other choice. As archaeologist Robert Liddiard explains, "Pitched battles, particularly in the eleventh and twelfth centuries, were rare."
Although his work was widely reproduced, and over 200 copies, translations, and extracts survive today, the extent to which Vegetius affected the actual practice of warfare as opposed to its concept is unclear because of his habit of stating the obvious. Historian Michael Clanchy noted "the medieval axiom that laymen are illiterate and its converse that clergy are literate", so it may be the case that few soldiers read Vegetius' work. While their Roman predecessors were well-educated and had been experienced in warfare, the European nobility of the early Medieval period were not renowned for their education, but from the 12th century, it became more common for them to read.
Some soldiers regarded the experience of warfare as more valuable than reading about it; for example, Geoffroi de Charny, a 14th century knight who wrote about warfare, recommended that his audience should learn by observing and asking advice from their superiors. While it is uncertain to what extent his work was read by the warrior class as opposed to the clergy, Vegetius remained prominent in the literature on warfare in the medieval period. In 1489, King Henry VII of England commissioned the translation of De re militari into English, "so every gentleman born to arms and all manner of men of war, captains, soldiers, vituallers and all others would know how they ought to behave in the feats of wars and battles".