Was there any military organization above a legion?

[mydogisfast]

When you think of roman military structure, you think Contubernium, Century, Cohort, and Legion. A legion consisting of a few thousand men, and led by a Legate. But I can’t seem to find any unit size above a legion, although it seems like they would have higher organizational levels seeing that there were quite a few legions. If no organizational structures, were there any formal positions above Legate? Or was that level of command reserved for the emperor/consul?

[caldrail]

No. It is quite common to see parallels with modern military organisation but a legion was not a regiment - it was an independent mini-army all of its own, a packet of military power to be distributed to where security was needed and in the hands of chosen commanders. There were various titles that in some circumstances could see you allocated command of a legion or two. Praetor, for instance. Even Quaestors (a high level finance minister) sometimes received commands if the situation demanded it. Prefects were usually at cohort level but the Praefectus Castrorum was senior enough to take command if his superiors were unavailable. The Praefectus Calssis commanded a fleet of ships. Praefectus Equitates was a cavalry commander. 

However, in the later empire the legions were re-organised to meet the demands of the day with smaller legions and more emphasis on cavalry. At the same time, permanent army organisations begin to appear  under the control of a Magister Militum, second only to the Emperor as military commander.

Edited October 19, 2021 by caldrail
typo

[caldrail]

The naval commander was Praefectus Classis. This site won't let me edit replies so I couldn't clear the typo. Sorry.

[Pompieus]

During the principate the governors of the military provinces (Upper and Lower Germany, Syria, Tarraconensis, Dalmatia, Moesia, Pannonia, Numidia) called legatus Augusti pro praetori, commanded all the legions and auxilia cohorts and alae stationed in their province. These were usually senators, ex-consuls or ex-praetors appointed by the Princeps. The exception was Egypt which was governed, and its army commanded, by an equestrian praefect. This was a flexible system and the Emperor sometimes combined several provinces and their armies under a single commander if necessary (often a member of the Imperial family)

As "caldrail' says Constantine created the Offices of Magister Equitum and Peditum to command subdivisions of the permanent field army he created (comitatus).  Diocletian (probably) had also created the rank of dux to command the border garrisons in groups of the smaller provinces he had created.

Edited October 25, 2021 by Pompieus

[caldrail]

It was based on a republican principle of temporary or limited assignment of military command. The right to command an army was called imperium and handed out to select individuals either as an honour or because it was necessary to find someone else to lead. The autocratic power of 'emperors' was based largely on having the highest level command privilege, imperium maius, which was one reason, aside from military kudos which the Romans had a deep attraction to, for the title 'Imperator' to be the most popular among those who were saluted as Rome's leader (even if they didn't actually get to take power). It is from 'Imperator' that we get the word 'Emperor'.

Note that a commander was supposed to surrender his imperium if he entered the Pomerium, the ritual heart of the city of Rome. Sometimes this was blatantly ignored - Sulla, Caesar, Severus, et al - or an inconvenience - Octavian/Augustus - but note also that the Senate awarded the right to command even though some emperors assumed it.

[Pompieus]

During the Republic a legion had no single commanding officer. The military tribunes rotated as a sort of "officer of the day".  An army commander (consul, praetor, proconsul or propraetor) possessing imperium, might assign one of his legati to control a legion if circumstances required, but only temporarily. The legatus legionis was an innovation of Augustus, created when the legions became permanent organizations.  The theory was the same as Augustus held proconsular imperium over all the provinces where legions were stationed.

[caldrail]

On the same lines, armies had two commanders, who swapped daily, the theory being that it made rebellion against Rome that much harder. They were supposed to be Consuls but as Rome expanded and the need to field armies in more places simultaneously then other offices were allocated.