Vegetius assigns one onager per cohort, transported on carts for camp defense and siege support [4]. The late empire favored these single-armed stone-throwers—brutal, simple, and movable—over delicate torsion frames. The wild ass kicked, but it also rolled.
What Happened
In Vegetius’ late-imperial establishment, the onager—a single-armed stone-thrower—appears one per cohort, ox-drawn and ready. This is not just a count. It’s an admission of preference in a changing army: fewer delicate twin-skein ballistae, more robust shock weapons that tolerate rough handling and deliver heavy stones where they must [4].
An onager rides to camp on a cart. The bed is braced; the arm is lashed down; the sling is tucked. At the next halt near Ravenna, artifices unload the frame and seat it on a leveled platform behind the ditch. When a raiding party tests the perimeter at dusk, the crew—eight winders plus a gunner, in Ammianus’ telling—crank the arm down against a padded stop, cushion the recoil, and let fly. The stone arcs, dark against an amber sky, and slams into a shielded knot with a thud you can feel in your ribs [3][4].
The reasons for the onager’s prominence are technical and administrative. A one-armed torsion thrower has fewer moving parts; it demands less precise maintenance of twin skeins; its ammunition—stones—can be sourced locally. In an empire where supply lines fray and skilled artifices are stretched thin, such economies matter. Vitruvius’ ratios still govern ballistae where they exist, but the late army needs a machine that forgives [1][3].
Mobility remains key. Vegetius’ per-cohort issue presumes carts, oxen, and teams trained to move and emplace with the speed of necessity. Trajan’s Column showed earlier generations how carts and frames integrate with march; the late cohort’s onager is that scene simplified and hardened [9][11]. The sound of it is distinctive: a single savage crack, followed by the deep tone of a heavy stone meeting earth or wall.
In Gaul, a cohort’s onager breaks a farmstead turned strongpoint on a Roman road. In Africa, it throws at a Berber hill fort while infantry clears a saddle. In the Balkans, it sits behind the trench, a camp guardian whose presence allows a commander to strip a watch from one sector to reinforce another. One engine per cohort is not abundance. It is coverage [4].
Ammianus’ nickname “onager”—wild ass—captures the recoil; Vegetius’ allocation captures the logic. Late Roman commanders accepted the kick in exchange for power and simplicity. The carts took the punishment of movement; the cushion took the punishment of release. The cohort got a fist it could close on short notice [3][4].
Why This Matters
Per-cohort onagers show a late-imperial army streamlining its siege arm without abandoning mobility. One engine per cohort spreads shock capability across the line and the map, ensuring that camps and columns can answer sudden problems with heavy stones [4].
The event fits the theme of late antique simplification. Against the delicate twin-skein ballista, the onager offers fewer parts, local ammunition, and tolerances for imperfect maintenance. Ammianus’ description of crews and cushions makes the trade-offs explicit [3].
In the larger arc, cohort onagers bridge Trajan’s mobile frames and the empire’s shrinking capacity. They are mobile, like the carts on the Column, but tuned to a leaner administrative reality. Vegetius’ paper issue reflects that compromise: keep firepower on wheels, but make it rugged [9][11][4].
For historians, the allocation helps explain archaeological patterns—fewer standardized bolt heads in late layers, more evidence of stone-throwing—and gives context to field deployments of engines behind the infantry line in late Roman battle doctrine.
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