Flavius Vegettos Renatus wrote one of the most important military works in the ancient world. Little is known of his life, other than that he wrote his 5 book collection in the 4th century and dedicated it to then emperor Theodosius the Great. He seems to have been a man of high social rank, but admittedly had no military experience. The 'Epitoma rei militaris' or 'De rei militari' is rather unprofessionally compiled and can be confusing, but is nevertheless an invaluable resource on the art of Roman warfare.
The first book paints a vivid picture of the state of the Roman army in the 4th century and is a plea to those in power to reform the decadence that prevailed. The second is a detailed description of officers and ranks in the legions. The third book is the most highly prized as it deals with tactics and strategy.
It was the basis for a military education during the Middle Ages and influenced a great many commanders of note, including Frederick the Great. The fourth and fifth books deal with siege warfare and the Roman navy respectively. Vegetius was translated into several languages, and the books printed for public consumption as early as the 15th century. With the accession of Napoleon and the advent of modern warfare over the next 2 centuries, Vegetius had fallen out of favor from a military education standpoint.