Vegetius Records Legionary Artillery Establishment

Vegetius is the sound of an army on paper. In his Epitoma Rei Militaris, likely compiled in the late 4th or early 5th century, he writes the numbers that make ancient machines feel modern: “In una autem legione quinquaginta quinque carroballistae esse solent. Item decem onagri…”—55 cart-mounted bolt-shooters per legion, one per century; 10 onagers, one per cohort [4].

Those numerals refract centuries of practice. Vitruvius gave the ratios that let a ballista exist in repeatable form [1]. Trajan’s Column carved carts into stone [9][11]. Vegetius binds them into establishment. He adds positioning, too: engines deployed “post aciem gravis armaturae,” behind the heavy infantry line, ready to contribute in open battle as well as in siege [4].

Picture a late-imperial camp outside Mediolanum. Lines of wagons sit in order; each cohort’s onager rests under a hide, its sling or cup greased; each century’s carroballista leans on a cart, skeins protected from damp. The din at midday is administrative: clerks calling rolls, artifices inventorying spare bolts. When a horn sounds, teams step to engines, mule bells tinkling as carts move to the rear of the drawn-up infantry line.

Vegetius’ prose blends the practical with the prescriptive. He knows he’s writing ideals. He is also clear about the crafts that make ideals viable: artifices—specialist soldiers—build testudines, rams, and “turres ambulatoriae,” walking towers [4]. The breadth of that list places artillery within a toolkit that includes earth and timber in equal measure. Caesar’s earth rings and Frontinus’ tricks find their bureaucratic heirs here [5][7].

The numbers imply reach. A legion with 55 carroballistae can deliver a dense hail of bolts wherever it finds cover. Ten onagers, ox-drawn, can set up to break gatehouses or deny approaches. Ammianus’ later description of onager crews—eight winders plus a gunner, cushions to ease recoil—sounds like the muscle behind Vegetius’ ink [3].

The places Vegetius imagines are everywhere and nowhere: fortified camps along the Danube, garrisons in Gaul, columns moving across Africa. His legions still look like legions. The color in his text is institutional—the pale parchment of a manual; the scarlet binding of an officer’s copy. But through his numbers we can hear the creak of carts and the crack of torsion in motion.

He wrote amid pressures. Administration strained; frontiers shifted; resources tightened. That context helps explain why onagers—simpler, one-armed stone-throwers—sit alongside ballistae in his table. The late empire still wanted reach, but it sought it with machines that tolerated rough handling [4][3]. Vegetius’ establishment is both a memory of Trajan’s confidence and a plan for survival.