From Maniple to Cohort

The basic unit of the Roman army was the legion, essentially a division of 4,500-5,000 men. The smallest unit of the legion was a century -- comprised of about 60-80 men. Each legion contained 60 centuries headed by a centurion.  Roman tradition dictated that the centurions be promoted from the ranks based on their courage, experience, initiative and skill.  Centurions were responsible for the training and conduct of the men beneath them. They combined the functions and prestige of a modern company commander and senior sergeant.

In the early days of the Republic, the centuries were paired into groups called maniples.  The legion's thirty maniples were then arranged, widely spaced, into three lines of ten maniples each.  In the first line, called the hastati, the men stood three deep and forty across.  The second line (principes) stood twelve wide by ten deep. The maniples of the third line were smaller than the first two and stood six wide and ten deep. In battle formation the hastati extended across the line of battle.  The principes and triari stood to the rear but not directly behind the hastati, instead they were offset diagonally creating a checkerboard effect.

The manipular formation was small enough to allow the army some articulation (the word maniple comes from the Latin for "hand," while phalanx derives from the Greek word for "finger") and some flexibility of movement.  Tactically, the units were small enough to maneuver on the battlefield with minimal training and were effective at flanking movements which the typical phalanx of the Alexandrian system could not match. Each maniple possessed its own set of standards, originally a handful of hay set on a pole, whose movements served for the transmission of orders.

The manipular legion served the Romans well for over two hundred years, but it suffered from tactical and organizational problems.  The same dispersion that allowed a maniple the flexibility to flank opponents, proved to be vulnerable to direct onslaughts. The small size of the maniple proved to be too "light" and a clever enemy could isolate and overwhelm a single maniple.  The other problem was that none of the legion's subdivision lent themselves to detachment for special duties.

Marius reorganized the legion by abolishing the maniple as a tactical unit, thus eliminating the last vestiges of the old phalangeal organization.  In its place he introduced a completely rational plan of articulation. He re-grouped the legion's centuries into ten cohorts of 500-600 men each.  This served to reconcentrate the legion's strength into larger units. Each cohort contained six centurions of different grades who provided the necessary continuity in the chain of command.

With the elimination of the maniples, Marius also eradicated the traditional three lines.  With cohorts as the basic operational unit, the legion�s men could be deployed into as few or as many lines as the circumstances warranted. The reform also reduced vulnerability to cavalry because its excellent articulation allowed parts of the army, legion or cohort, to maneuver to protect a flank, and the good subdivision permitted the commander to assign a unit or units to flank protection. Since formed heavy infantry could stop the charge of formed heavy cavalry, Roman commanders felt little anxiety about their flanks with this setup.

At the same time, Marius (and those who followed) clarified the command structure.  From the overall commander of the army through the legates, military tribunes, centurions and common soldier, every commander had full authority over the men in the unit he commanded.  He, in turn, was answerable to a commander over him. The Roman practice of subdivision and subordination had another advantage -- it gave real scope for the initiative of subordinates on the battlefield.  Battlefield communications were so poor once battle was joined that the role of the general was primarily one of determining when to commit the reserve.  For that reason Roman commanders harangued their troops before combat. The speech was not only for morale boosting but to give the men a general outline of the overall plan so that they could execute it without a steady stream of orders. More than once a legion had been saved when centurions or tribunes did what they were expected to do without a general in earshot.  Furthermore, while normally considered to be a tactical alteration, the cohort system also gave a commander units with enough strength to defend themselves and that could easily be separated from the rest of the legion to carry out special duties.  Marius thus created a fully articulated army capable of maneuvering and responding promptly to the orders of its leaders.

On the battlefield, cohorts assumed a formation about five hundred feet wide by fifty feet deep.  In another departure from the manipular formation, the cohort formed a continuous front while engaged in hand to hand fighting. In action, the lines of the cohort rotated, allowing a fresh line to appear at regular intervals of about 10 minutes. The engaged line withdrew and fell to rear until its turn came again. No other opponent operated in this fashion, hence an attacking force was continuously faced with relatively fresh troops during the entire battle. Eventually the enemy could no longer stand the strain and would succumb to exhaustion. If they broke, the hasty disorganized retreat usually turned into a fatal rout.

The first real test of the Marian tactical system came at Aix-En- Provence when a combined army of Ambrones and Teutones, numbering around 130,000 warriors confronted Marius consular army of six legions. The battle was one-sided and resulted in the extermination of both tribes -- Marius took 17,000 warriors prisoner and another 130,000 dependents.  The following year, at Vercellae in the Po valley, Marius defeated the Cimbri, killing 140,000 and enslaving 60,000 warriors, and taking a like number of women and children prisoners.  The German threat was removed.  Marius was elected consul for the 6th time, and hailed the "Third Founder of Rome."  The Marian tactical system became the model for the rest of Rome's history.