Who commanded a legionary or auxiliary cohort?

[LonginusXXI]

Who commanded a legionary or auxiliary cohort? Was it the centurion in command of the first century, the pilus prior a.k.a. the centurio princeps as the author in the historical novel The Eagle of the Ninth says? Or was it a tribunus?

[Gunthamund Hasding]

ok
auxilia and legions are different.

legions we simply don't know, theories are the rank higher centurion in the cohors but could be an one of the tribunii militum as well. Some say it had no clear standing commander.

Auxilia is clear. Augustus time was an praefectus, who could be either a native nobleman, who would probably be granted Roman citizenship for the purpose e.g. the famous German war leader Arminius gained Roman citizenship probably by serving as an auxiliary prefect or a Roman, either of knightly rank, or a senior centurion.

The Tres militia were shaping during Claudius times and later we would have the following picture


Roman auxiliary regiments: Type, structure and strength
Ala quingenaria
cavalry
praefectus
decurio
16 turmae
30 (32)
480 (512)

Ala milliaria
cavalry
praefectus
decurio
24 turmae
30 (32)
720 (768)

Cohors quingenaria
infantry
praefectus*
centurio
6 centuriae
80
480

Cohors milliaria
infantry
tribunus militum
centurio
10 centuriae
80
800

Cohors equitata quingenaria
infantry plus
cavalry contingent
praefectus
centurio (inf)
decurio (cav)
6 centuriae
4 turmae
80
30.
600
(480 inf/120 cav)

Cohors equitata milliaria
infantry plus
cavalry contingent
tribunus militum
centurio (inf)
decurio (cav)
10 centuriae
8 turmae
80
30
1,040
(800 inf/240 cav)

[Nathan Ross]

As Gelu says, auxiliary cohorts were commanded by equestrian officers - prefects for 500-man cohorts, tribunes for double strength (milliaria) or citizen cohorts.

Legion cohorts appear to have had no overall commander. You might look through this long and very detailed thread for some opinions on that question:

Cohort Commander?