Cohort commander?

[Renatus]

In 423 BC Livy describes a decurion leading a cavalry cohort into battle. With an infantry cohort made up of six centuries and with five cavalry allocated to each century, a cavalry cohort equates to 30 men. Now because there are 10 cohorts to a legion, there can be 10 cohorts of cavalry each at 30 men. Therefore, an ancient historian is correct in labelling 30 cavalrymen a squadron or a cohort.

I assume that you are thinking of Livy 4.38.1-3 and 4.39.1. The first reads:

Nihil nec imperium nec maiestas valebat, dataque mox terga hostibus forent, ni Sex. Tempanius, decurio equitum, labante iam re praesenti animo subvenisset. Qui cum magna voce exclamasset ut equites qui salvam rem publicam vellent esse ex equis desilirent, omnium turmarum equitibus velut ad consulis imperium motis,"Nisi haec" inquit, "parmata cohors sistit impetum hostium, actum de imperio est . . .”

and translates as:

'There was no virtue either in his authority or in his dignity; and his men would presently have shown the enemy their backs, had not a cavalry decurion named Sextus Tempanius, just as the situation was becoming desperate, come with prompt courage to the rescue. In a loud voice he cried out that the horsemen who wished to save the state should leap down from their horses, and when the troopers in every squadron had bestirred themselves as if at the command of the consul, he added: "Unless this bucklered cohort stops the enemy's rush it is all over with our supremacy . . .”' (Loeb translation)

The second reads:

Et cum iam parte nulla sustinerentur, dat signum Volscus imperator ut parmatis, novae cohorti hostium, locus detur, donec impetu inlati ab suis excludantur.

and translates as:

'When the Volscian general saw that their attack could not anywhere be stopped, he ordered his troops to give ground to the men with bucklers, the enemy's new cohort, until, carried forward in their rush, they should be cut off from their friends.' (Loeb translation)

It seems clear that this decurion did not command a cavalry cohort as such but that the so-called cohort was an ad hoc grouping of men from all turmae fighting as infantry. "Cohort" here, therefore, is probably used generically for a band of men, rather than as a technical term. Turma appears to be the appropriate term for a troop of horse. In the second passage, "cohort" may be used in a technical sense, although it could be rhetorical, but in context it refers to men fighting as infantry, not cavalry.

It would be dangerous to assume from this that there was any such thing as a cavalry cohort or to ascribe any particular number of men to it.