Vegetius

Late Roman military writer whose Epitoma Rei Militaris (Military Institutions of the Romans) was the primary military manual of the European Middle Ages — cited by commanders from Charlemagne to Napoleon — preserving Roman military doctrine when the legions had collapsed.

Preserved Roman military knowledge for a thousand years after Rome's fall — the emphasis on training, logistics, and discipline rather than heroism — making Vegetius the military authority that every medieval European commander read.