Vexillatio (plural vexillationes) was a unit separated from the parent legion, most often for a specific purpose, created during the principate’s times. It took its name most likely from vexillum, a legionary mark.
Contrary to popular belief, it seems that vexillationes were made by auxilia soldiers, not legionaries. The term ward appears in the singular, but the term in the plural was used more often due to the fact that such wards operated in groups. Vexillationes were established ad hoc in crisis situations at the borders. There were many such situations in the 3rd century CE, during the great crisis of the Roman Empire: usurpations, rebellions, and invasions of neighbouring countries/peoples. The vexillatio unit differed both in number and composition, but as a rule, it amounted to about 1000 infantry soldiers and 500 horsemen.
The reasons for the creation of vexillationes and their application stemmed from the difficult period in which the Roman Empire found itself in the 3rd century CE. A huge Roman army, dating back to the beginning of the 3rd century CE around 400,000 people was unable to dynamically respond to all threats and incidents across wide borders. Rarely was it possible to send an entire legion to the site of a rebellion or invasion, and there were no reserves as such. The only logical solution was to separate the troops from the legions and merge them into a single unit to eliminate the threat. Immediately after completing the quest, the unit split and returned to its home legion.
Other troops such as: infantry – cohortes and centuriae; and rides – alae and turmae, could have had their own vexilla. In turn, the vexillatio unit itself, having separate vexilla units, could use them as special units. They had functions similar to vexillarii – restored veterans.
The vexillatio system initially functioned very well, due to mobility (due to the great road system) and high discipline, consistency and morale. However, during the “Great Depression of the 3rd century CE” (from 235 to about 290 CE), the vexillationes were so often moved from place to place that there was a great confusion of troops and disorganization of the Roman army. To this end, the successive emperors Diocletian and Constantine the Great had to carry out military reforms to strengthen the structure of the army.
During the time of the dominion, the term vexillatio referred to a horse unit in the Roman army. The Vexillationes were probably used as cavalrymen in many campaigns from the time of Diocletian and the introduction of the tetrarchy. In the 4th century CE vexillationes palatinae and vexilationes comitatenses were supposed to be between 300 and 600 people. The Roman document from the end of the empire Notitia Dignitatum mentions the existence of 88 units of vexillationes.