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A vexillatio was a detachment of a Roman legion
Roman legion
A Roman legion normally indicates the basic ancient Roman army unit recruited specifically from Roman citizens. The organization of legions varied greatly over time but they were typically composed of perhaps 5,000 soldiers, divided into maniples and later into "cohorts"...

 formed as a temporary task force created by the Roman Army
Military history of ancient Rome
From its origin as a city-state in Italy in the 8th century BC, to its rise as an empire covering much of Southern Europe, Western Europe, Near East and North Africa and fall in the 5th century AD, the political history of Ancient Rome was typically closely entwined with its military history...

 of the Principate
The Principate is the first period of the Roman Empire, extending from the beginning of the reign of Caesar Augustus to the Crisis of the Third Century, after which it was replaced with the Dominate. The Principate is characterized by a concerted effort on the part of the Emperors to preserve the...

. It was named from the standards carried by legionary detachments, vexillum (plural vexilla), which bore the emblem and name of the parent legion. Although commonly associated with legions, it is likely that vexillationes included Auxiliaries
An auxiliary force is a group affiliated with, but not part of, a military or police organization. In some cases, auxiliaries are armed forces operating in the same manner as regular soldiers...

. The term is found in the singular, referring to a single detachment, but is usually used in the plural to refer to an army made up of picked detachments. Vexillationes were assembled ad hoc
Ad hoc
Ad hoc is a Latin phrase meaning "for this". It generally signifies a solution designed for a specific problem or task, non-generalizable, and not intended to be able to be adapted to other purposes. Compare A priori....

 to meet a crisis on Rome's extensive frontiers, to fight in a civil war
Civil war
A civil war is a war between organized groups within the same nation state or republic, or, less commonly, between two countries created from a formerly-united nation state....

, or to undertake an offensive against Rome's neighbours. They varied in size and composition, but usually consisted of about 1000 infantry
Infantrymen are soldiers who are specifically trained for the role of fighting on foot to engage the enemy face to face and have historically borne the brunt of the casualties of combat in wars. As the oldest branch of combat arms, they are the backbone of armies...

 and/or 500 cavalry
Cavalry or horsemen were soldiers or warriors who fought mounted on horseback. Cavalry were historically the third oldest and the most mobile of the combat arms...


Because the Roman Army
Roman army
The Roman army is the generic term for the terrestrial armed forces deployed by the kingdom of Rome , the Roman Republic , the Roman Empire and its successor, the Byzantine empire...

 was not large enough to garrison the vast size of the empire (around 400,000 strong at the beginning of the 3rd century) most of it was stationed along the frontiers. This placed the empire in a precarious position when serious threats arose in the interior or along a remote frontier. There was no central reserve and it was rarely possible to take a full legion, or even a major portion of one, to a troubled area without leaving a dangerous gap in the frontier defences. The only logical solution was to take detachments from different legions and form temporary task forces to deal with the threat. As soon as it was taken care of, these vexillationes were dissolved, and the detachments returned to their parent legions.

The vexillatio system worked initially, due to the mobility provided by the empire's excellent roads and to the high levels of discipline, cohesion and esprit de corps of these units and the legions from which they came. But during the Crisis of the Third Century
Crisis of the Third Century
The Crisis of the Third Century was a period in which the Roman Empire nearly collapsed under the combined pressures of invasion, civil war, plague, and economic depression...

 (a turbulent period from 235 to about 290) vexillationes were shifted so rapidly from one area to another that units became hopelessly mixed up. This was a major cause of disorganization in the Roman Army which resulted in sweeping military reforms under Diocletian and Constantine I
Constantine I
Constantine the Great , also known as Constantine I or Saint Constantine, was Roman Emperor from 306 to 337. Well known for being the first Roman emperor to convert to Christianity, Constantine and co-Emperor Licinius issued the Edict of Milan in 313, which proclaimed religious tolerance of all...


Later, under the Dominate
The Dominate was the "despotic" latter phase of government in the ancient Roman Empire from the conclusion of the Third Century Crisis of 235–284 until the formal date of the collapse of the Western Empire in AD 476. It followed the period known as the Principate...

, vexillatio refers to a cavalry unit of the Roman army. From the time of Diocletian
Diocletian |latinized]] upon his accession to Diocletian . c. 22 December 244  – 3 December 311), was a Roman Emperor from 284 to 305....

 and the Tetrarchy
The term Tetrarchy describes any system of government where power is divided among four individuals, but usually refers to the tetrarchy instituted by Roman Emperor Diocletian in 293, marking the end of the Crisis of the Third Century and the recovery of the Roman Empire...

, and possibly as early as the reign of Gallienus
Gallienus was Roman Emperor with his father Valerian from 253 to 260, and alone from 260 to 268. He took control of the Empire at a time when it was undergoing great crisis...

, vexillationes were the usual cavalry units found on campaign. In the 4th century the Vexillationes palatinae
Palatini (Roman military)
The palatini were elite regiments of the Late Roman army mostly attached to the comitatus praesentales, or imperial escort armies...

and Vexilationes comitatenses
Comitatenses is the Latin plural of comitatensis, originally the adjective derived from comitatus , itself rooting in Comes .However, historically it became the accepted name for...

of the Roman field armies are thought to have been either 300 or 600 men strong. The Notitia Dignitatum
Notitia Dignitatum
The Notitia Dignitatum is a unique document of the Roman imperial chanceries. One of the very few surviving documents of Roman government, it details the administrative organisation of the eastern and western empires, listing several thousand offices from the imperial court down to the provincial...

lists 88 vexillationes.

Other units, such as infantry cohortes and centuriae, and cavalry alae and turmae, may have had their own vexilla. In addition, vexillationes with their own vexilla would have designated companies of special troops outside the usual military structure, such as vexillarii (re-enlisted veterans), who may have served separately from the cohorts of their ordinary comrades.