Vexillatio

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.

Augustus created the praetorians who at the time of Domitianus constituted a force of ten cohorts, each of 1000 men strength on paper and who supplied a disputed number of horsemen.

[1][2] 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 and became practically independent.

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 though the ala remained.

In addition, vexillationes with their own vexilla would have designated units 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.