Because the Roman Army 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 (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.

Later, under the Dominate, vexillatio refers to a cavalry unit of the Roman army. From the time of Diocletian and the Tetrarchy, and possibly as early as the reign of Gallienus, vexillationes were the usual cavalry units found on campaign. In the 4th century the Vexillationes palatinae and Vexilationes comitatenses of the Roman field armies are thought to have been either 300 or 600 men strong. The Notitia Dignitatum lists 88 vexillationes.

References

• R.E. Dupuy and T.N. Dupuy, The Encyclopedia Of Military History: From 3500 B.C. To The Present. (2nd Revised Edition 1986), pp 147-148.
• Pat Southern and Karen Dixon, The Late Roman Army (1996), chapter 2. ISBN 0-415-22296-6