In 1985, Professor M.P. SPEIDEL noted that 'the structure and command of vexillations is still largely
unknown.,1 Since then, a number of scholars have advanced our knowledge of particular vexillations, but we are still very much in the dark with regard to even the elementary aspects of the origins, methods of creation, organization, command and service of these units in general. The universally accepted English translation of the word vexillatio is 'a detachment', and, according to the established view, the employment of such bodies was an innovation of the Emperor Augustus, who, after settling the legions and auxihary units in permanent garrisons around the Empire, devised the vexillatio as a means of quickly re-deploying portions of armies or units to deal with emergencies. Detachments, it is widely believed, could range in size from a small handful of men through to a force of several thousand, and, when taken from the legions, were normally commanded by senatorial officers, that is up until the time of the Marcomannic
wars, when the pressures of that conflict necessitated the employment of equestrian officers in the same role. However, a close examination of the evidence reveals that the word 'detachment' is a misleading, and at times incorrect, translation of vexillatio. In truth, a vexillatio was a new (albeit temporary) unit, usually (though not exclusively) structured on the model of a regular unit and/or sub-units, with a new title and a new commander, who (even from the late first century BC) was most often an equestrian officer, a decurion or a centurion. The origins of the vexillatio, moreover, go back well beyond the reign of Augustus, at least as far back, in fact, as the Second Punic War of the late-third century BC. Though vexilla are seldom recorded in our sources for the Republic, the wealth of evidence for these types of units in the Principate has revealed a great many details about how soldiers were
selected for service in a vexillatio; what the strengths and organization of single vexillations and larger special commands were, and the ranks of those who led them, as well as a number of aspects concerned with their operational employment.
1M.P. Speidel, 'A Pannonian Optio Vexillationis Buried at Stratonikeia', Epigraphica Anatolica 6 (1985),75=Mavors 8,67.