Vexillatio

A vexillatio (plural vexillationes) was a detachment of a Roman legion formed as a temporary task force created by the Roman army of the Principate. It was named from the standard carried by legionary detachments, the 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. 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 to meet a crisis on Rome's extensive frontiers, to fight in a civil war, or to undertake an offensive against Rome's neighbours. They varied in size and composition, but usually consisted of about 1000 infantry and/or 500 cavalry.

Purpose

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.