“Acies quadratum” On the square formation of the Roman army

[Nathan Ross]

Pompey <advanced with his line of march> in a squared formation… Pompeius quadrato <agmine procedit>***

whether it be needful for the troops to draw into a square... seu sit opus quadratum acies consistat in agmen,

I wonder if there might be a difference here between acies quadratum and agmen quadratum, the first being a battlefield formation and the second a march formation? Obviously the second was intended to facilitate deploying into the first, and any Roman march column could presumably deploy into line of battle quite quickly.

A couple of descriptions of Roman 'defensive' march formations come to mind, one from Herodian and one from Tacitus. Neither, I think, uses the phrase agmen quadratum, but that seems to be what they are describing:

Herodian (8.1.2-3): Leading his army down into level country, Maximinus drew up the legions in a broad, shallow rectangle in order to occupy most of the plain; he placed all the heavy baggage, supplies, and wagons in the center of the formation and, taking command of the rear guard, followed with his troops. On each flank marched the squadrons of armed cavalry, the Moorish javelin men, and the archers from the East.

And in this formation 'the troops... crossed the plain in good order and strict discipline'.

Tacitus (Annals I.51): [Caesar] took the road prepared either to march or to fight. A detachment of cavalry and ten auxiliary cohorts led the way, then came the first legion; the baggage-train was in the centre; the twenty-first legion guarded the left flank; the fifth, the right; the twentieth held the rear, and the rest of the allies followed.

In both cases the formation seems to be a widened column rather than a square as such - the intention being to protect the baggage train against attacks from any side. Obviously it would only be possible to keep this sort of formation in relatively open country - like the arid plain that the Crusaders would be operating in, I expect. It's interesting that Tacitus describes the enemy attacking only when "the whole line was defiling through the wood: then instituting a half-serious attack on the front and flanks, they threw their full force on the rear."

Advancing over any distance in full battlefield formation (the acies quadratum) would be very difficult, then. The terminology is perhaps hazy, but agmen quadratum was not a battlefield formation, I would suggest, but a defensive march formation designed to enable rapid deployment if required.

The criteria is from all directions, and it should be mandatory for all modern historians describing this event to show all four.

In most cases it would quite obvious to any commander which direction the enemy was attacking from, taking into consideration possibilities of outflanking. No need, surely, to assume a formation facing in all four directions - that way, three quarters of your fighting strength would be rendered useless against the main attack?