Maniples and Cohorts

The Roman army used the phalanx as a attack formation until they realized the futility of the phalanx on un-level ground during a battle with the Samnites in 315 B.C. The consuls decided to create a more flexible system as a result the manipular formation was born. Later, the manipular system was replaced by the cohort system.

The drawing above compares the manipular system with the cohort system.

In Republican times, the Roman army marched toward the enemy in three lines Hastati, Principes, and Triarii. A legion would contain 10 maniples of each of the three types as its infantry component.

The cohort system was adopted at least partly in response to the difficulty fighting barbarians. The Celts and Spaniards were able to defeat a highly trained army because of their concentrated charges. The way to counter them was to concentrate men, in formation, on the Roman side: building cohorts out of maniples.

The date of the transition to the cohort formation is disputed. Conventional wisdom says to credit Marius for the innovation because he built Rome’s first professional army. But Marius never claimed he developed the cohort and we know enough details of his life to have that information. In addition, there are historical documents describing the use of cohorts long before Marius. There are seventeen references to cohorts by Livy writing about the period 210-195 B.C. So the debate is out there without a resolution. Let’s leave this issue and move on to discuss the cohort formation itself.

To be honest, we don’t know much about it and what we do know is controversial.

We know that a cohort contained six centuries of 80 men or 480 men. Ten cohorts made up the infantry portion of the imperial legion. It seems the cohorts contained Hastati, Principes, and Triarii (also called Pili) within them so that the unit had a mixture of experienced and inexperienced troops. Some drawings depict the positions of the Pili and Hastati reversed compared to Republican times (Pili in the lead), but this is disputed. It seems unlikely to me that the Romans would reverse their attack philosophy after perfecting it over seven centuries. Why put the most experienced troops out front and leave raw recruits as reserves?

My opinion is supported by what one historian calls the “fossilization” of Roman design, meaning that the Romans changed the purpose or behavior of a structure but always stuck to its traditional design. Examples: the Hastati were probably named for the “hasta” thrusting spear but in the imperial period they carried a pilum and gladius. The principes where probably the “prime” force originally but in historical times they are in the second position.

The drawing above lists the names of the six centurions assigned to the cohort. The “prior” centurion is on the right and the “posterior” on the left facing the enemy – each leading his century. I have not shown the first cohort which had a unique structure of five double centuries for a total of 800 men. The senior centurion, in command of the first cohort, was called the Primus Pilus.

Despite the cohort structure, there is reason to believe that the infantry types and smaller groupings were still retained by the Roman army. For example, Hadrian once had separate meetings with Hastati, Principes, and Triarii under his command to give them specific instructions. He would not have done so if the cohort was the overriding formation. There is also a building inscription recording the work of the fourth Hastatus maniple of the second legion on Hadrian’s Wall.