Poetovio was an important fortress on the River Drava which controlled the approaches to Italy through the Julian Alps[c] It cannot be determined whether Aper commanded the fortress garrison or whether he answered to a superior officer who would probably have been styled as a dux[d] The force under Aper's command would have consisted of elements (i.e. vexillationes) of the legions mentioned rather than the legions themselves.
Such units were independent of the regular command-structures of the frontier garrisons as traditionally deployed reflecting the strategic reaction of the imperial government to the anarchic situation in the Danube provinces (and also, incidentally, in Mesopotamia and Syria) caused by incessant incursions by northern barbarians (and the forces of the Persian Empire) into Roman territory and savage civil conflicts which were in large measure a consequence of the failure of the imperial government to control such incursions.
The new strategy which relied on ad hoc mobile expeditionary forces brought about a great expansion of the command-opportunities for officers of equestrian as opposed to senatorial rank.
[5] Aper is also identified[6] with the officer commemorated on an undated epigraph from Aquincum in Pannonia Inferior (modern Budapest, Hungary).
[g] There is no way of knowing the specific circumstances that had led the Imperial Authorities to give Aper this posting, but the most likely reason was that the local situation required a man with military experience and that no suitable senatorial could be found.
Historian Pat Southern, points to Aper's scheming as the most likely reason for Carus's unexpected death while campaigning against the Sasanian Empire.
[1] What is incontestable is that when Numerian (who was by that time the Emperor following the death of his father) died as the Imperial comitatus returned from its victorious campaign in Persia.
The suspicion of murder evidently arose because Aper had attempted to conceal the fact of Numerian's death, perhaps while he prepared the ground for his own accession to the Purple.
[15][h] Flavius Vopiscus relates that Diocletian did this to fulfill a prophecy which had been delivered to him by a female druid, "Imperator eris, cum Aprum occideris.
"[17][18][19] The historian Edward Gibbon was to say of this episode: A charge supported by such decisive proof was admitted without contradiction and the legions with repeated acclamations acknowledged the justice and authority of the Emperor Diocletian.