The Roman army used the phalanx as a attack formation untilthey realized the futility of the phalanx on un-level ground during a battlewith the Samnites in 315 B.C. The consuls decided to create a more flexiblesystem as a result the manipular formation was born. Later, the manipular systemwas replaced by the cohort system.
The drawing above compares the manipular system with thecohort system.
In Republican times, the Roman army marched toward the enemyin three lines Hastati, Principes, and Triarii. A legion would contain 10maniples of each of the three types as its infantry component.
The cohort system was adopted at least partly in response tothe difficulty fighting barbarians. The Celts and Spaniards were able to defeata highly trained army because of their concentrated charges. The way to counterthem was to concentrate men, in formation, on the Roman side: building cohortsout of maniples.
The date of the transition to the cohort formation isdisputed. Conventional wisdom says to credit Marius for the innovation becausehe built Rome’s first professional army. But Marius never claimed he developedthe cohort and we know enough details of his life to have that information. Inaddition, there are historical documents describing the use of cohorts longbefore Marius. There are seventeen references to cohorts by Livy writing aboutthe period 210-195 B.C. So the debate is out there without a resolution. Let’s leavethis issue and move on to discuss the cohort formation itself.
To be honest, we don’t know much about it and what we doknow is controversial.
To be honest, we don’t know much about it and what we doknow is controversial.
We know that a cohort contained six centuries of 80 men or480 men. Ten cohorts made up the infantry portion of the imperial legion. Itseems the cohorts contained Hastati, Principes, and Triarii (also called Pili) withinthem so that the unit had a mixture of experienced and inexperienced troops.Some drawings depict the positions of the Pili and Hastati reversed compared toRepublican times (Pili in the lead), but this is disputed. It seems unlikely tome that the Romans would reverse their attack philosophy after perfecting itover seven centuries. Why put the most experienced troops out front and leaveraw recruits as reserves?
My opinion is supported by what one historian calls the“fossilization” of Roman design, meaning that the Romans changed the purpose orbehavior of a structure but always stuck to its traditional design. Examples:the Hastati were probably named for the “hasta” thrusting spear but in theimperial 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 centurionsassigned 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 notshown the first cohort which had a unique structure of five double centuriesfor 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 thatthe infantry types and smaller groupings were still retained by the Roman army.For example, Hadrian once had separate meetings with Hastati, Principes, andTriarii under his command to give them specific instructions. He would not havedone so if the cohort was the overriding formation. There is also a buildinginscription recording the work of the fourth Hastatus maniple of the secondlegion on Hadrian’s Wall.
