The Roman legion, the largest military unit of the Roman army, comprised 4,200 infantry and 300 equites (cavalry) in the period of the Roman Republic and 5,600 infantry and 200 auxilia in the period of the Roman Empire.
The Praetorian Guard was an elite unit of the Imperial Roman army that served as personal bodyguards and intelligence agents for the Roman emperors.
A cohort was a standard tactical military unit of a Roman legion. Although the standard size changed with time and situation, it was generally composed of 480 soldiers. A cohort is considered to be the equivalent of a modern military battalion. The cohort replaced the maniple. From the late second century BC and until the middle of the third century AD, ten cohorts made up a legion. Cohorts were named "first cohort", "second cohort", etc. The first cohort consisted of experienced legionaries, while the legionaries in the tenth cohort were less experienced.
The equites constituted the second of the property-based classes of ancient Rome, ranking below the senatorial class. A member of the equestrian order was known as an eques.
The Roman army was the armed forces deployed by the Romans throughout the duration of Ancient Rome, from the Roman Kingdom to the Roman Republic and the Roman Empire, and its medieval continuation, the Eastern Roman Empire. It is thus a term that may span approximately 2,205 years, during which the Roman armed forces underwent numerous permutations in size, composition, organisation, equipment and tactics, while conserving a core of lasting traditions.
Praefectus, often with a further qualification, was the formal title of many, fairly low to high-ranking, military or civil officials in the Roman Empire, whose authority was not embodied in their person but conferred by delegation from a higher authority. They did have some authority in their prefecture such as controlling prisons and in civil administration.
The limitanei, meaning respectively "the soldiers in frontier districts" or "the soldiers on the riverbank", were an important part of the late Roman and early Byzantine army after the reorganizations of the late 3rd and early 4th centuries. The limitanei, unlike the comitātēnsēs, palātīnī, and scholæ, garrisoned fortifications along the borders of the Roman Empire and were not normally expected to fight far from their fortifications.
The auxilia were introduced as non-citizen troops attached to the citizen legions by Augustus after his reorganisation of the Imperial Roman army from 27 BC. By the 2nd century, the Auxilia contained the same number of infantry as the legions and, in addition, provided almost all of the Roman army's cavalry and more specialised troops. The auxilia thus represented three-fifths of Rome's regular land forces at that time. Like their legionary counterparts, auxiliary recruits were mostly volunteers, not conscripts.
The structural history of the Roman military concerns the major transformations in the organization and constitution of ancient Rome's armed forces, "the most effective and long-lived military institution known to history." At the highest level of structure, the forces were split into the Roman army and the Roman navy, although these two branches were less distinct than in many modern national defense forces. Within the top levels of both army and navy, structural changes occurred as a result of both positive military reform and organic structural evolution. These changes can be divided into four distinct phases.
In modern scholarship, the "late" period of the Roman army begins with the accession of the Emperor Diocletian in AD 284, and ends in 480 with the death of Julius Nepos, being roughly coterminous with the Dominate. During the period 395–476, the army of the Roman Empire's western half progressively disintegrated, while its counterpart in the East, known as the East Roman army remained largely intact in size and structure until the reign of Justinian I.
Aper was a Roman citizen of the third century AD. First known to history as a professional soldier, he went on to serve as an acting provincial governor and finally became Praetorian prefect, under the Emperor Carus - in effect "vice principis". This rendered him hugely influential in the government of the empire - not excepting in matters of peace and war.
The palatini were elite units of the Late Roman army mostly attached to the comitatus praesentales, or imperial escort armies. In the elaborate hierarchy of troop-grades, the palatini ranked below the scholares, but above the comitatenses and the limitanei.
A turma, was a cavalry unit in the Roman army of the Republic and Empire. In the Byzantine Empire, it became applied to the larger, regiment-sized military-administrative divisions of a thema. The word is often translated as "squadron" but so is the term ala, a unit that was made up of several turmae.
The equites singulares Augusti were the cavalry arm of the Praetorian Guard during the Principate period of imperial Rome. Based in Rome, they escorted the Roman emperor whenever he left the city on a campaign or on tours of the provinces. The equites singulares Augusti were a highly trained unit dedicated to protecting the emperor. Men who served in the equites singulares Augusti held a Roman public status as equites.
The Imperial Roman army was the military land force of the Roman Empire from 27 BC to 476 AD, and the final incarnation in the long history of the Roman army. This period is sometimes split into the Principate and the Dominate (284–476) periods.
Traianus Mucianus was a Roman soldier of Thracian origins of the second half of the Third Century AD who rose from the lowest ranks of the army to senior commands. He was almost certainly a remarkable soldier. However, the successive promotions he secured in the latter part of his career are thought to owe much also to the favour shown him by men highly placed in the Imperial entourage whose patronage secured him advantageous postings in the Imperial comitatus, the mobile field force under the direct command of the Emperor, that was undergoing massive expansion at this time.
The equites Dalmatae were a class of cavalry in the Late Roman army. They were one of several categories of cavalry unit or vexillatio created between the 260s and 290s as part of a poorly understood reorganization and expansion of Roman cavalry forces. These new cavalry vexillationes, typically bearing the basic regimental designation equites, mostly originated as detachments of existing mounted units, which were separated from their parent corps and later constituted as independent units.
By the size of the Roman army is meant the changes in the number of its contingents: legions, auxiliaries, Praetorian cohorts, Urban cohorts, vigiles, and naval forces over the course of twelve centuries – from 753 BC to AD 476.
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