Assembling the Army


The army leaves its camp as a large number of century-sized units moving out of each gate in several columns. They have to be assemble themselves into the ccohorts, turmae, alae and legions of the army. Moreover, the legions have to be in the proper order since specific units were assigned places in the line of battle based on the tactics of the day. The cavalry might be placed on one or another flank, or both; the light infantry had to find its assigned place, and the various support units needed to come together. All of this could only be done outside the walls of the camp.

This page will explore this process.


The only actual battle that these pages have examined in depth is Phasalus. A portion of that battle map shown on the right. Caesar's army in this battle was roughly the same size as the army described in the section The Army on the March. That page gives a general idea of how Caesar's army at Pharsalus might have moved on the road. Some of the parameters of the Battle of Pharsalus can serve as a kind of template for considering just what might have been involved in getting an army from camp to battle -- a very different operation from the open-road march.

What will be used from the Pharsalus example are just two items: the general distances involved and the major terrain feature, the river.

The actual distance between the two camps is about 2.5 miles, the distance from Caesar's camp to the line of battle is 8,689 feet, a little over 1.5 miles. If we take these distances as representative of Roman field practices it would seem that an army would have between 1 and 2 miles to travel from camp to battle.

The significant terrain feature is that the ground between camps is generally open and relatively unobstructed. Overnight marching camps may have been located in well protected locations where access was limited on several sides. But these were camps that were not constructed as bases for major battles. When the army set up a camp at an expected site of battle it selected a site with a broad open area on one side at least. The ground between the camp and the selected site of the forthcoming battle would be as open as possible both to facilitate the attack but also to make retreat back to the camp easier. The most common thing would be that the army could move from camp to battle over a fairly broad front and in a relatively straight line.

Based on the above analysis, the model for the camp, assembly area and battlefield are shown in the illustration on the right. Every situation would be different but this template serves well enough as a generic representation.

The overall distance from the camp to the eventual site of the battle is modeled at 1.5 miles. Since camps were used as refuges in case of defeat, it is unlikely that a commander would wish to be more than two miles from his own camp. Some commanders tried to stage the battles much closer -- Pompey offered battle to Caesar while his army was still within range of his camp artillery -- several hundred yards.

A key factor is that the area between the camp and the battlefield is considered to be relatively open and level. There is no need for it to be perfectly flat for the army could not possibly move more than a few hundred yards in battle formation without breaking up. But the ground is considered to be open enough to allow several columns of marchers to move across it in parallel lines.

Phase One: Assembly

The first elements to exit the camp would be the cavalry and the light infantry. They would act as scouts and as protective screens for the army as it leaves the camp.

The next step in the process is for the centuries in their columns of two's to leave the camp and assemble themselves into their cohorts and legions somewhere outside the camp. A location fairly close to the camp has been selected for this process. Although the centuries could certainly march all the way to the eventual battlefield in their columns, the army would be unacceptably vulnerable for much too long a time. And, since we do know that at least in some cases armies actually arrayed in battle formation quite close to their camps, it is reasonable to think that they would normally re-assemble the cohorts close to camp as well.

The following illustration shows the general routes for the columns of two's to follow from the camp gates to the points of assembly for each legion.

The arrows show general routes the columns could follow. Since Legion IV ( the magenta arrow) exits through two side gates, one of its columns must cross over other columns. This could create traffic problems, however Legions I and II have shorter distances to travel and could pass in front of the centuries of Legion IV.

Since we know that each legion has a vexillum, a banner on a pole with the legion's number on it, once could speculate that these vexilla might have been used to mark the assembly points for each legion.

Red: Legion I
Green: Legion II
Blue: Legion III
Magenta: Legion IV
Dark Green: Cavalry

The two lines within the oval on the right of the drawing show the length of the columns of one ala of cavalry (green) and one cohort of infantry.

In the illustration below the cavalry and light infantry are shown in dark green on either flank. In practice, by the time the rest of the army assembled, those units would be further afield performing their defensive and scouting funcitons.

Of interest is the shape, size and location of the assembly formations. Compared to a battle or even a parade formation, they are narrow and deep. The reason for this becomes clear in the illustration below which is a detail of the circled area.

This shows three legions in assembly formation. The only difference is that the wide gaps between them have been reduced to make the illustration a bit clearer. It shows three legions in various stages of assembly.

Legion III (blue) is shown in the process of assembling. The four separate columns-of-two's can be distinguished at the bottom right. Two or three of the leading centuries in each of the columns is highlighted with a red arrow. These centuries are still in motion. 15 centuries are already in place in the first line, four in the second and 6 in the third. These are the centuries arranged side by side and without arrows. There is room between the forming cohorts for the columns of centuries to pass through. The final position of the lead century of each of the four columns is shown in the sets of paralle lines. Because the centuries may not exit the camp in perfect order, this formation allows them to file into their correct positions within the legion in any sequence.

Legion II (red) is fully assembled. The centuries retain their column-of-two's structure, hence the formation is narrow but deep.

Legion I (green) has begun stepping out on its march toward the final battlefield. Since the ground between the assembly location and the battlefield is realtively open, the legion can move in 12 parallel columns.

Since the army continues in columns-of-two's one might wonder why the assembly step is useful. There are two reasons. If the first place, it would be difficult, nearly impossible, to ensure that every century exits the camp in precisely the correct sequence. The pause to assemble assures that all of the centures are in their correct place before the army moves any closer to the enemy. The second reason is that the legions can probably only march in 4 parallel columns to the assembly point but could very well move in as many as 12 or even 24 columns from there to the battlefield. And they could arrive at the battlefield with every century in its correct location. This would make the second stage of moving much faster and greatly reduce the vulnerability of the army to enemy attack.

The final step in the process of deploying the army is to move from the assembly location to the battlefield and then to arrange the units in the battle formation. These steps are considered next.

Links to the five parts of the Deployment section:
Deploying the Army: Planning
Deploying the Army: Exiting the Camp
Deploying the Army: Assembling the Units
Deploying the Army: Marching to the Battlefield
Deploying the Army: Forming in Battle Formation

Site Map

2003, Gary Brueggeman. All rights reserved world wide. No part of this work may be reproduced in part or whole, in any form or by any means, without permission from the author.