Cohort System

  • Introduced into the Roman Legions in 107 BCE, the Cohort consisted of 480 men divided into 6 centuries of 80 men, replacing the Maniple System.

Introduction

  • After 107 BCE, the Cohort System replaced the Maniple System, under the Marian Reforms.
  • The Maniple System faced the enemy in three rows of 20 Maniples, with the new recruits and youngest soldiers forming the first row. Marius removed the first row and inserted the inexperienced units amongst the experienced units.
  • Marius then concentrated the Legion’s 60 Maniples of 120 men (7,200 men) into 10 Cohorts of 480 men, each divided into 6 centuries of 80 men (5,
  • The Cohorts were numbered one to ten, ‘Cohors I’, ‘Cohors II’ to ‘Cohors X’.
  • As in the Maniple System, the Century remained the basic unit, but with 80, instead of 60 men in each Century.With 6 Centuries in each Cohort, there were now 6 Centurions of varying grades, giving a strong vertical Command Structure. This in itself, made the Cohort a much more powerful unit than the Maniple.

How were the Cohorts organised?

  • Cohors I:
    • 5 Double strength Centuries (160 men in each) – 800 men.
    • Commanded by the Primus Pilus – the most senior Centurion of the Legion.
    • The Elite and most senior Cohort of the Legion, with the most experienced soldiers.
  • Cohors II:
    • 6 Centuries (80 men in each) – 480 men.
    • Commanded by the Pilus Prior.
    • One of the four weaker Cohorts (II, IV, VII and IX) of less experienced men.
  • Cohors III:
    • 6 Centuries (80 men in each) – 480 men.
    • Commanded by the Pilus Prior.
  • Cohors IV:
    • 6 Centuries (80 men in each) – 480 men.
    • Commanded by the Pilus Prior.
    • One of the four weaker Cohorts (II, IV, VII and IX) of less experienced men.
  • Cohors V:
    • 6 Centuries (80 men in each) – 480 men.
    • Commanded by the Pilus Prior.
  • Cohors VI:
    • 6 Centuries (80 men in each) – 480 men.
    • Commanded by the Pilus Prior.
  • Cohors VII:
    • 6 Centuries (80 men in each) – 480 men.
    • Commanded by the Pilus Prior.
    • One of the four weaker Cohorts (II, IV, VII and IX) of less experienced men.
  • Cohors VIII:
    • 6 Centuries (80 men in each) – 480 men.
    • Commanded by the Pilus Prior.
  • Cohors IX:
    • 6 Centuries (80 men in each) – 480 men.
    • Commanded by the Pilus Prior.
    • One of the four weaker Cohorts (II, IV, VII and IX) of less experienced men.
  • Cohors X:
    • 6 Centuries (80 men in each) – 480 men.
    • Commanded by the Pilus Prior.
  • Cavalry Unit (Eques Legionis)
    • In addition, there would be a 120 man cavalry unit.
  • Total number of Men in the Legion:
    • 5,240 men in the Legion, excluding Officers.

An Example of how the 10 Cohorts could be lined up:

  • Two rows of five, one behind the other.
    • The double strength First Cohort (I) always positioned on the right.
    • The 6 senior cohorts of experienced men on the flanks and centre:
    • The 4 less experienced cohorts inbetween:
      front
      V IV III II I
      X IX VIII VII VI
      cavalry

Tactics of the Cohort

  • Complete Flexibility
    • The Three lines of the Maniple System, the Hastati, the Principes and the Triarii were all abolished.
    • The 10 Cohorts could now be deployed to face the Enemy in:
      • two lines (5 + 5).
      • or in one long line ( 10 )
      • or in three lines (4 + 3 + 3)
      • the last being a formation apparently favoured by Julius Caesar during the Gallic Wars.
    • Cohorts could now be moved around tactically, during the actual Battle.
    • Cohorts could be moved out to protect the flanks, or moved in to block a penetration.
    • They could be rotated to face a threat from a new direction, or from behind.
  • Withstanding Frontal Assaults
    • Due to its larger size, the Cohort could better withstand a full frontal assault, whereas individual Maniples had been vulnerable to a head on assault due to their smaller size.
  • Withstanding Cavalry Attacks
    • The Cohort was less vulnerable to a Cavalry attack on the Flanks.
    • The Cohort could realign its infantry lines rapidly to successfully resist a light or heavy Cavalry charge.
    • The whole Cohort could rotate to meet a Cavalry attack from a new direction (Rear or Flank attack).
  • Independent chain of Command
    • The Cohort gave its commanders sufficient strength in numbers for it to act as an independent unit. The junior officers could now make their own decisions and enforce them, during the course of the Battle.
    • The Roman Generals would give an outline of the Plan to their Commanders before the Battle. It was then up to the junior officers to implement the Plan.
    • During a Battle, communications often collapsed, and with the Maniple System the General’s Strategy was basically restricted to committing the Reserves.
    • With the Cohort System, the General outlined his Stategy to the Officers before the Battle. This way, even if the Cohorts became isolated from the central command, their Officers could continue to make and implement their own decisions, based on the original Plan. This transformed the effectiveness of the Legion on the Battlefield.
  • Independent Detachments: the ‘Vexillatio’
    • Cohorts could also now be sent away on special duties as a separate detachment, the Vexillatio, because of this independent command structure.

107 BCE
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