With Sasanian King Shapur’s death, the Zoroastrian priesthood seized its chance to stifle what it saw as a potent threat to its social and religious dominance. The minorities of Varahran I (273–6) and Varahran II (276–93) seem to have been dominated by the Mazdean priesthood on the one hand and the Persian nobility on the other. It is during these reigns that we find the otherwise unknown phenomenon of a non-royal figure (Kardir again) inscribing his exploits in a public and royal context. It was also now that Mani was arrested and left to die in prison, perhaps in 276. Then, in the 280s, there was a civil war between supporters of Varahran II and those of his cousin Ohrmazd. The latter relied upon nomadic Sakas and levies from the Kushanshahr for support, minted his own coinage and may have even taken the title of Kushanshah. The war between the cousins also divided the empire’s upper nobility, and the great families of both Persian and Parthian background now began to demonstrate an attitude they would maintain right until the collapse of the Sasanian dynasty: while the right of the Sasanian family to rule went unchallenged, the nobility reserved their right to choose among potential royal claimants, and depose one Sasanian in favour of another should that seem necessary. For the first, but not last, time in the history of the two empires, a combination of internal religious ferment and external distraction on Persia’s eastern frontiers made the 270s and 280s a period during which Rome had little to fear from the armies of its imperial rival.
Perhaps realising this – though with the caveat that Roman military intelligence on Persian affairs was never very comprehensive – Carus marched into Mesopotamia in the summer of 283, straight down the Euphrates to the capital at Ctesiphon. The campaign went so well that word was put about that Ctesiphon had actually fallen to the emperor, and one strand of sources preserves that story. But Seleucia-on-Tigris, by then a suburb of Ctesiphon, was indeed sacked, as attested by Ammianus Marcellinus, a good fourth-century source, who had inspected the ruins with his own eyes when serving on the emperor Julian’s Persian campaign.
Carus’s military success was not rewarded by his army’s loyalty: he was murdered, like so many of his predecessors, though there is also a story, surely legendary, that his tent was struck by lightning. His murder left the army stranded deep inside Persia, and the first order of business was to extricate it. Whoever was responsible for Carus’s death, no one claimed his title, which passed to his sons Carinus and Numerian, the latter a young boy who had accompanied his father’s campaign army into Persia. Carus’s brother-in-law Aper, who had succeeded him as praetorian prefect, probably took de facto control of affairs until the army was back in Syria. Disarray among the Persians had helped them to extract the army intact and largely unscathed – Aper and his officers were lucky that no new Shapur emerged to harass them on their retreat.
The Roman forces reached Emesa in Syria by March 284, and Cyzicus and Nicomedia in Bithynia later in the year. There, in November, Aper announced the death of Numerian. Although the boy had been ill, his death was almost certainly murder. There was now just one emperor, Carinus back in the west, but Carus’s campaign army was not likely to accept its subordination to a western rival. As Carus’s brother-in-law, Aper believed he should succeed to the throne, but the army did not concur. Instead, its choice landed on a relatively junior officer, C. Valerius Diocles, a man of about forty who was the comes domesticorum, commander of the main guard unit which travelled with the emperor. Diocles accepted his acclamation, and renamed himself Diocletianus, a more Latinate-sounding name than his obviously Greek (and low-born) original. At a meeting of the whole army, he personally executed Aper in full public view, claiming thereby to avenge the murder of Numerian. He then proclaimed himself consul prior, along with L. Caesonius Bassus, a member of the Roman senatorial aristocracy. The truth of what happened can never be known. Numerian may have died naturally, with both Aper and Diocletian seeking to profit by his death, or one of them may have killed him. (We do not need to believe the story that, for some time after his murder, his corpse had been carried alongside the army in a litter in order to disguise it, word being put about about that he was suffering from a disease of the eyes.) A child emperor with no protector was a victim awaiting the slaughter, his death a foregone conclusion, but Diocletian’s acclamation at Nicomedia necessarily meant civil war. Carus’s remaining son, the emperor Carinus, could not be expected to acquiesce in the eastern army’s presumption and, by taking the consular title for himself and appointing a colleague, Diocletian effectively declared war on Carinus.
Diocletian overwintered in Asia, before crossing into the Balkans, with an eye on attacking Carinus. The latter marched east from Rome, knowing that he would need to face Diocletian in the Balkans, but he had none of the personal authority his father Carus had commanded, and he had no dynastic prospects as his son Nigrinianus had died. The praetorian prefect Sabinus Iulianus, left behind by Carus to watch over his son, now revolted, presumably viewing him as unlikely to survive a war with the seasoned army of Diocletian. This revolt was put down at Verona, but then the corrector Venetiae, Marcus Aurelius Julianus, revolted in Pannonia, striking coins at Siscia before being defeated by Carinus early in 285 as he marched to the Balkans. There, at the Margus river, his army faced its more formidable rival, and Carinus’s newly appointed praetorian prefect, Tiberius Claudius Aurelius Aristobulus, promptly betrayed him as well. So too did M. Flavius Constantius, who was then serving in an extended military command over Dalmatia and the Balkan interior. Constantius had probably previously served as a tribune of the domestici under Diocletian; he would go on to be a major prop of the latter’s new regime. As we have seen many times, third-century soldiers were quick to desert a commander whose prospects looked dim, so Carinus was promptly assassinated by his own men. He and his wife Magnia Urbica suffered damnatio memoriae, as did Carus and Numerian, their names being chiselled from inscriptions.
The battle of the Margus ended in a relatively easy victory for Diocletian, but the past decades had shown that initial success counted for very little without persistent labour. Diocletian marched his army into northern Italy and took up residence at Mediolanum. There he made another general of no great pedigree his colleague as caesar, on 25 July 285. This general was Marcus Aurelius Maximianus, the son of a shopkeeper from very near Sirmium whose family had gained citizenship only with the edict of Caracalla. Diocletian and Maximian had come up through the ranks together and both had been present on Carus’s Persian campaign. Neither man troubled to go to Rome at this point – there were too many problems on the northern frontiers. By the autumn of 285, Maximian was campaigning in Gaul, where the death of Carinus had prompted a revolt by a man named Amandus, who proclaimed himself augustus. Carus was from Narbonensis and Amandus may have been a relative; later sources also name one Aelianus as a part of the revolt, but he remains a mystery, as no authentic coins in his name have survived, only modern fakes. Maximian seems to have suppressed the revolt fairly efficiently, and later tradition reinvented Amandus and Aelianus as rustic brigands rather than the provincial notables they were. Still, their uprising prompted the usual uncertainty on the frontier, and Maximian also fought against Franci or Alamanni from across the Rhine. Diocletian, at the same time, was fighting against Sarmatians on the Danube bend, now a perpetual sore point trapped between the empire and what we infer was Gothic power expanding up the Danube and in the former province of Dacia. We do not know where Diocletian overwintered in 285–6, but he was back in Asia Minor in March of 286. Gaul was far too unsettled for Maximian to leave it.
The expedient of having an augustus in one place and a caesar of similar age and experience in another was a novelty. No one, least of all his soldiers, could have expected Maximian to remain in a subordinate role for ever. On 1 April 286, he was duly proclaimed augustus, without Diocletian being present, but with his full, stated approval. The cynical interpretation would be that Diocletian gave his old comrade something he would obviously have wanted before he took it for himself and thereby sparked a civil war; the loftier and more noble reading would have Diocletian and Maximian embarking on a bold experiment in power-sharing, the better for each to hang on to the imperial thrones they had won. What is certain is that they, or Diocletian, now began to concoct an elaborate ideological system to explain what everyone could see was an unprecedented relationship. Diocletian began to style himself as the child or companion of that most Roman of gods, Jupiter, the chief of the Capitoline triad, and thus the greatest god in the Roman pantheon. Maximian, for his part, was to be Hercules, Jupiter’s son and loyal subordinate. Both emperors were equally augusti, both augusti were equally divine, but Diocletian was the senior, as Jupiter was senior to Hercules. This articulated a traditional Roman paternalism but injected the issue of divine election into the question of who could be emperor. Here we can see how far things had changed since the Antonine age: a hundred years before Diocletian, Commodus had been mercilessly ridiculed for his self-assimilation to Hercules; fifty years earlier, Elagabalus’s belief in his own divine incarnation led directly to his assassination. By contrast, when Aurelian advertised his personal relationship with the unconquered sun it was accepted as perfectly reasonable, as, so it seems, was Diocletian and Maximian’s identification with Jupiter and Hercules.
It remained to be seen whether this bold experiment in power-sharing between two men with no dynastic connection would work, especially because Maximian had a son on the threshold of adulthood, while Diocletian had only daughters. The presence of a presumptive heir to the junior augustus must have complicated expectations about both the present and the future balance of power. In the moment, though, both men had plenty of work to do.
In 286, a general named Carausius revolted in Britain, declared himself augustus and began minting coins. A Menapian by birth, from the territory between the Rhine and Scheldt, he had been the commander of the Channel fleet, protecting the coastline from Saxon and Frankish pirates. His revolt was a serious concern: not only the armies of Britain, but also many stationed in Gaul itself took his side, and he began striking coins at Rotomagus (modern Rouen) as well. Maximian was too busy fighting on the frontiers of eastern Gaul throughout the later 280s to do anything about this revolt, which is surprising given that usurpations almost axiomatically trumped barbarian incursions on the scale of imperial threats. The fact that Maximian divided his time among the main cities of the Rhineland – Trier, Moguntiacum and Colonia Agrippina – may suggest that he doubted his ability to challenge the usurper successfully, and it may be that a wide tranche of western Gaul recognised Carausius rather than the new regime of Diocletian and Maximian. Rather than face the usurpation outright, Maximian launched a police action on the frontier to shore up his authority, crossing the Rhine, sowing terror among the barbarians and installing a king named Gennobaudes among some of the Franci there. The victory emboldened him and, in 288, he led an army against Carausius, winning a battle at Rotomagus and regaining control over north-western Gaul. He then began to construct a fleet, taking the better part of a year to do so, only to see it destroyed in a North Sea gale before he could launch an invasion. Carausius promptly retook the Gallic towns he had recently lost to Maximian. It must have seemed like the fatal rhythm of the mid third century was returning.
While Maximian was in Gaul, Diocletian was fighting in the Balkans, with journeys to Syria in 286 and 287 to observe developments in Persia, which had been ignored for more than a decade apart from the brief campaign of Carus. Aurelian’s destruction of rebellious Palmyra had destabilised politics on the Syrian frontier, for in 289–90 we find Diocletian at the old caravan city fighting the desert tribes and also visiting the important Severan site of Emesa. Further north, Shapur’s weak successors had lost control of Armenia, and Diocletian was able to install as king the Arsacid Tiridates III, who had fled to the Romans as a child decades earlier.
On the whole, Diocletian had been having much greater success in his sphere of activity than Maximian in his. In 290, Diocletian inspected the field armies in the Balkans, en route to northern Italy, where he met Maximian in the winter of 290–91. Whatever else it may have been – and there was no love lost between the two augusti, however much they might need one another – the meeting at Mediolanum was a show of unity. The two received embassies from the cities of the west and from the senate at Rome. This reaffirmed the tradition by which earlier soldier emperors had coopted the support of the imperial capital’s aristocracy, although neither Diocletian nor Maximian showed the slightest inclination to go there. Despite the symbolic power that Rome and its senate still commanded, sometime in the quarter century between Gallienus and Diocletian, the traditional link between emperor and eternal city had been comprehensively broken. In fact, there was no longer really an imperial capital, just a series of residences of greater or lesser importance: Trier, Mediolanum, Nicomedia and Sirmium were the most important at this stage in Diocletian and Maximian’s joint reign, but there would be others. The outlines of the old, high imperial world were growing very blurred, and we – with the full benefit of hindsight – can see a new late imperial order beginning to take shape.
Shortly after the meeting in Milan, Maximian resigned the command against Carausius. It was taken over by a subordinate general, the same Flavius Constantius whose timely betrayal of Carinus eased Diocletian’s path to power in 284 – and who had already been rewarded by marriage to Maximian’s daughter Theodora in 289. The events of later 291 and all of 292 are obscure in the extreme, almost as opaque to the historian as the reign of Probus, but when the sources resume in 293, it is with a series of momentous, indeed unprecedented, reforms. Perhaps these were agreed upon at Mediolanum in 291; perhaps Diocletian spent the intervening years working out how he might shore up a regime that remained shaky even with two cooperative joint rulers at its head. What emerged at the start of 293 was nothing short of extraordinary – having discovered that even two emperors were not enough to secure a stable regime, and having quite unaccountably survived longer than any emperor since Gallienus, Diocletian and his colleague overhauled the shape of imperial government – from the currency to the army, to the administration of the provinces and even to the imperial office itself. On 1 March 293, two new caesars were appointed, and the shared emperorship of Diocletian and Maximian became an imperial college of four members, two senior augusti and two junior caesars. With the creation of this tetrarchy, a new stage in Roman history begins, one that reinvented the very nature of the empire.