De Re Militari (Latin "On military matters") was a treatise of late Roman warfare that became a military guide in the middle ages.

Writer of De Re Militari was Flavius Vegetius Renatus who lived in the late 4th century. It may be that Vegetius wrote it trying to formulate a military reform , possibly under the patronage of Theodosius I. It has been also called Epitoma Rei Militaris. The phrase si vis pacem para bellum ("If you want peace, prepare for war") probably originates from Vegetius.

Vegetius based his treatise on Roman armies. He emphasized things like training of soldiers as a disciplined force, orderly strategy, maintenance of supply lines, quality leadership and use of tactics and even deceit to ensure advantage over the opposition. He was concerned about selection of good soldiers and recommended hard training of at least four months before the soldier was accepted into the ranks. The leader of the army (dux or duke) had to take care of the men under his command and keep himself informed about the movements of the enemy to gain advantage in the battle.

De Re Mlitari became a popular book about warfare in the middle ages, especially between 9th and 16th centuries, even if some of the information did not suit to later times. Especially Vegetius' notes about siegecraft became obsolete when the technology advanced. Vegetius' suggestion of a soldier's religious oath to God and to the realm might have influenced knightly practices. Still, because of the lack of literacy, as a guide it was probably accessible only to aristocracy, clergy and royalty.

226 Latin copies of the book have survived to the modern day, not including translations to various other European languages. Many of them have copious amount of personal notes on them, pointing at matters that have interested their contemporary owners.

External links

• E-text of English translation of De Re Militari
• Homepage of a society of De Re Militari that is concerned about medieval warfare