But what if there was no such enmity between Constantina and Constantius? What if she was not acting as a kingmaker in her own right, but merely saw the elevation of Vetranio as a way to preserve the power of her own family? And what if Constantius was in on it too? This is perhaps the most famous explanation of the peculiar circumstances and events of the usurpation of Vetranio: that the whole scenario was a plot by imperial loyalists to thwart Magnentius’ aim to assume control of the Balkans and ultimately retain the region for the Constantinian dynasty, with Constantina, Vetranio and Constantius all involved in its planning and execution.
There are certainly aspects of usurpation which raise the prospect of imperial collusion. The timing of the revolt would certainly prove opportune for Constantius, keeping Magnentius out of Illyricum and eventually delivering the Illyrian field army to the emperor when he could not be on hand due to the siege of Nisibis. There is Constantina’s involvement and then not being relegated from the corridors of imperial power in the aftermath. Vetranio’s previous loyalty to the Constantinian dynasty undermining the idea that he would strike out completely on his own. The rapidity of Constantius’ ‘acceptance’ of Vetranio’s elevation, sending him cash and a diadem could hint at orchestration. That Vetranio and/or Constantius moved to meet at Serdica perhaps even without their armies followed by what might be described as an orchestrated acclamation and sudden abdication at Naissus may also indicate collusion. The respectful tone Constantius used towards Vetranio after his abdication, both at Naissus and with his retirement to Prusa, may demonstrate that the emperor had organised this retirement for a man loyal enough to risk usurpation. There also appears to have been a general lack of punitive action towards the Illyrian army and Balkan hierarchies after the usurpation, unlike that which Constantius carried out in Gaul, Britain and the east through Paul ‘the Chain’ (Crawford (2016), 83-84, 101, 128, 176-177).
It might be asked why would Vetranio need to take the title of Augustus if he was in any way still loyal to Constantius? The answer was in the fickleness of soldiery. In a straight choice between a man recognised as Augustus in half of the empire and a magister peditum, the soldiery could not be trusted to make the ‘right’ choice. It has been suggested that Diocletian had only elevated Maximian to Augustus because the British usurper, Carausius, had taken the Augustan title for himself, and the disparity in perceived imperial status could have undermined Maximian’s attempts to recover Britain. In a similar vein, Vetranio assuming the title of Augustus not only put him on the same level as Magnentius, as already seen it also gave him the opportunity to bolster the loyalty of his men by doling out an accession donative.
However, while this idea of a well-orchestrated loyalist plot involving the magister peditum, his advising Augusta and the legitimate ruling Augustus might seem interesting, it is almost certainly not correct. There was likely some anti-Magnentian sentiment and possibly some hint of Constantinian loyalism in Vetranio’s decision to assume the imperial title, particularly given the involvement of Constantina, but Constantius was surely not involved from the start. First and foremost, “such a flagrant manipulation of the imperial title would seem beyond a man who held it in such high esteem” (Crawford (2016), 76).
There is also the complete lack of available time for Constantius to be directly involved. When Vetranio usurped the imperial title on 1 March 350, it had only been 43 days since Magnentius had himself usurped the position at Autun. Was there really time for word to reach Constantius in Edessa, for him to then correspond with Constantina and Vetranio and for them to plan a loyalist usurpation for it to be executed by 1 March? The likelihood is that Constantius was completely in the dark not just about Vetranio’s usurpation but perhaps even about that of Magnentius and the murder of Constans by the time Constantina’s letter arrived. Indeed, why would Constantina need to send word to Constantius explaining what had happened with regard to Vetranio, if he been part of the planning?
The emperor’s involvement in the plot was therefore merely his acceptance of whatever explanations Constantina and Vetranio provided for their actions. His sending of financial aid, permission for Vetranio to assume command of the Danubian forces and a diadem in recognition of the Illyrian magister’s new imperial status was a pragmatic acceptance of the fait accompli presented to him and a recognition that the Illyrian usurper might be useful to him. Constantius will also have recognised the potential disaster inherent in rejecting the arrangement come to by Vetranio and Constantina. Vetranio’s willingness to find an accommodation might not last. He had already shown himself willing to treat with Magnentius with the joint embassy that implored Constantius to accept a three-way division of power and there may have been other worrying “signs of rebelling,” (Philotorgius III.22) such as the fortifying of his position not just in the Julian Alps against Magnentius but also to the east in the direction of Constantius (Philotorgius III.24). The ardent rejection that likely came immediately to mind for Constantius may have thrust Vetranio into the arms of Magnentius, making that ambassadorial arrangement a military alliance against Constantius. It is perhaps here that the presence of Constantina bore the most fruit, acting like a go-between for emperor and usurper and helping her brother not reject Vetranio as soon as he heard he was calling himself Augustus.
The correspondence from Constantina and the professions of fealty from Vetranio allowed Constantius to present himself as being in some sort of control. He likely found it propagandistically useful to have people think that Vetranio had only assumed the imperial title on his authority. It plays into an idea that he could project power from the farthest eastern extremities of the empire into its central provinces.
But all of that pragmatism, accommodation and compromise might be for naught if Constantius dithered in the east. Even a loyal man such as Vetranio could be tempted or persuaded to wield his usurped imperial authority either in opposition to Constantius and in doing something rash like bringing Magnentius to battle. Such negative developments had to be completely shutdown and the only way to do that was for Constantius to be there in person. Thankfully for him, the walls and garrison of Nisibis proven beyond Shapur II, allowing Constantius to leave the east to his generals and march quickly to the Balkans in time for Christmas.
Faced with such a potent combination, Vetranio set aside any imperial aspirations he had and reverted (if he had ever deviated from them) to his long history as a Constantinian loyalist in return for a comfortable retirement on the Black Sea coast. He had played his part: a loyal man perhaps forced to assume an imperial title by circumstances beyond his control (even if he had had his head turned by the lure of power for the briefest moment) - the advance and attempts at bribery of Magnentius, potential rumours of usurpation within his own ranks and the influence of Constantina, who was either scheming for her own ends or recognised the gravity of the situation playing out in the Balkans and felt that her intervention would be best for the imperial family, even if it meant encouraging a non-Constantinian to assume the imperial mantle. For both Constantina and Vetranio, usurpation might have been the ultimate show of Constantinian loyalty.
Whatever the circumstances of his usurpation, Vetranio had facilitated Constantius’ bloodless claiming of Illyricum in the face of Magnentius’ advance, which the emperor may well have thought worthy of respect and peaceful retirement. Ultimately, “the story behind Vetranio’s brief stint as emperor is somewhere in the middle of a stage-managed usurpation and out-and-out rebellion:” (Crawford (2016), 76) it began as the latter, with a slight hint of Constantinian influence, and essentially became the former due to Vetranio’s previous loyalty, Constantius’ pragmatism and rapid arrival in the Balkans.
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