Flavius Vegetius Renatus

Publius (or Flavius) Vegetius Renatus, known as Vegetius (Latin: [u̯ɛˈɡɛtiʊs]), was a writer of the Later Roman Empire (late 4th century). Nothing is known of his life or station beyond what is contained in his two surviving works: Epitoma rei militaris (also referred to as De re militari), and the lesser-known Digesta Artis Mulomedicinae, a guide to veterinary medicine. He identifies himself in the opening of his work Epitoma rei militaris as a Christian.

The latest event alluded to in his Epitoma rei militaris is the death of Emperor Gratian (383); the earliest attestation of the work is a subscription by Flavius Eutropius, written in Constantinople in 450, which appears in one of two families of manuscripts, suggesting that a division of the manuscript tradition had already occurred. Despite Eutropius' location in Constantinople, the scholarly consensus is that Vegetius wrote in the Western Roman Empire. 

Vegetius dedicates his work to the reigning emperor, who is identified as Theodosius, ad Theodosium imperatorem, in the manuscript family that was not edited in 450; the identity is disputed: some scholars identify him with Theodosius the Great, while others follow Otto Seeck and identify him with the later Valentinian III, dating the work to 430–35. Goffart agrees that the later date is likely, suggesting that the work may have been intended to support a military revival in the time of Aetius's supremacy. 

Rosenbaum also argues that he wrote in the early 430s; Theodosius II might then have been the dedicatee. Rosenbaum uses allusions from Vegetius's works and relationships to the work of Merobaudes to suggest that Vegetius was a senior court official, primiscrinius to the praetorian prefect, who had been an agent in rebus.

Vegetius' epitome mainly focuses on military organization and how to react to certain occasions in war. Vegetius explains how one should fortify and organize a camp, how to train troops, how to handle undisciplined troops, how to handle a battle engagement, how to march, formation gauge, and many other useful methods of promoting organization and valor in the legion.

As G. R. Watson observes, Vegetius' Epitoma "is the only ancient manual of Roman military institutions to have survived intact." Despite this, Watson doubts its value. Vegetius "was neither a historian nor a soldier: his work is a compilation carelessly constructed from material of all ages, a congeries of inconsistencies." These antiquarian sources, according to his own statement, were Cato the Elder, Cornelius Celsus, Frontinus, Paternus, and the imperial constitutions of Augustus, Trajan, and Hadrian.

The first book is a plea for army reform; it vividly portrays the military decadence of the Late Roman Empire. Vegetius also describes in detail the organization, training, and equipment of the army of the early Empire. The third book contains a series of military maxims, which were (appropriately enough, considering the similarity in the military conditions of the two ages) the foundation of military learning for every European commander from William the Silent to Frederick the Great.

His book on siegecraft contains the best description of the Late Empire and Medieval siege machines. Among other things, it shows details of the siege engine called the onager, which afterward played a great part in sieges until the development of modern cannonry. The fifth book gives an account of the materiel and personnel of the Roman navy.