Cohort commander?

[Mark Hygate]

Since Polybius came up in the discussion I have to point you to his account that in each legion there were 2 centurions for every speira but also 6 chiliarchoi for every legion. Were they 10, we would be able to speak about possible commanders of cohorts but 6? Maybe cohorts did not have a single commander but somehow the command of any detachment of one or more cohorts were indeed commanded by them?......

Yes, 2 Centurions for every maniple and 6 Tribunes for each legion (or Prefects for the Socii). In the 'standard legion' the Triarii maniple is only a 'century' in strength, however, so there are the equivalent of 50 centuries. What's missing from that equation then is the cavalry contingent of 10 Turmae (Roman legion) and now there are 60 sub-units to command/administer - 10 for each Tribune.

When it comes to the 'need for a cohort' question, however, and the subsequent command of such; most logically linked to the times and needs of smaller units than whole legions/consular armies and certainly it seems by the post-Marian Citizen legions (single) and continuing in the Roman tradition of dealing in those original 10s; let alone the double-cohort sized structure of the Extarordinarii in Early Republican times; then the cohort of '5' (and then fixed at 6) centuries (complete with their traditional centurion rank structure in place) is born.

Then, considering simply the OPs original question for the post-Augustan period....

For the legions it is certainly reasonable to consider that the individual cohorts were commanded by the senior centurion of each one; but that Tribunes were assigned when 'standard' vexillations of pairs of cohorts were detached, partially for historical traditional reasons and the need for a more 'senior' command function.

The larger millaria auxilia cohorts (still of 10 sub-units) were also commanded by Tribunes.

For the legions it seems unlikely that the Tribunes had fixed command duties in camp, given what we generally seem to understand of the political structure (perhaps cynically ). In terms of military command and discipline it seems much more likely that the day-to-day administration was carried out by the Camp Prefect and his 10 senior centurions and down what more modern armies might now consider the 'Sgt's Mess chain'.

Tribunes probably had 'much more important' things to do. 8)