The new emperor, Diocletian, was yet another former Danubian general who was to prove to be as outstanding an administrator as his predecessors had been military exponents. Diocletian realized early on that the empire simply could not any longer be handled by one man; whenever he was in any one part of it, pretenders arose or barbarians invaded in another. Carausius, given a fleet to combat the pirate menace in the North Sea, had fled with it in 286 to Britain. The Bagaudae tribe had rebelled once the former emperor Carinus had left Gaul, and barbarians had invaded in 286. Therefore he divided the empire into two (286), appointing an old army friend and general, Maximian, as his coemperor in the west; Diocletian took the wealthier east. Both in turn appointed successors as emperors designate, known as caesars, to provide a system of rule known as the Tetrarchy (293). It meant that the most capable generals were assured a share of the imperial power, while the presence of effectively four rulers enabled a much closer guarding of the frontiers and posed an almost insuperable problem to any usurpers. The tetrarchs built magnificent palaces for themselves in their self-selected capitals of government. They also launched a savage new attack (303–304) on the fast-growing Christian Church that would prove to be both the last and also the fiercest of all the Church’s persecutions, and would persist in the eastern empire until 311.

Diocletian raised the manpower of the army from some 300,000–400,000 in the time of Severus to more than half a million men under arms, and again favoured the concept of a frontier defence where barbarians were to be checked at the empire’s perimeter and not within it. He continued the separation of the frontier garrisons, comprising mostly ex-barbarian soldiers, from the mobile ‘rapid-response’ units of infantry and cavalry that were generally made up from conscripts from the empire reinforced with barbarian special troops. This rapid expansion of the army must have caused a deterioration in the quality of the recruits.

Diocletian also reorganized the provinces into dioceses for better management and defence. The number of provinces within each diocese was approximately doubled from the original number, with extra provincial governors and their administrations, partly with a view to making rebellion harder for would-be usurpers. Each provincial governor now commanded about half the number of soldiers that he might have called upon to support a rebellion before the division of the provinces.

The Tetrarchy also had the effect that the numbers of senior administrative staff were perhaps quadrupled, the dioceses doubled the number of civil servants and the army had also been greatly increased. All had to be paid for; taxation was changed from an erratic collection when needed to a regular levy. Aurelius Victor claims that the new tax burden to pay for all this had been fair as first imposed by Diocletian, but that later emperors became greedy. The Christian writer Lactantius, a contemporary of Diocletian, paints a grim picture of the cost of the new bureaucracy and armies.

Diocletian tried again to reform the currency after Aurelian’s partial success in the 270s. He reminted the gold coin at sixty to a Roman pound of gold and a high-grade silver coin was issued at ninety-six to the pound weight of silver. Aurelian’s old reformed antoninianus was standardized now at 3 per cent silver content, including face wash, and about 10 grams weight. The name of the new coin is unknown; it is today referred to by numismatists as a follis, although a very similar coin described in the ancient sources as a nummus appears in the fourth century. Again, the silver content of the follis drifted down over succeeding years to about 1.5 per cent silver before Constantine the Great introduced new reforms. Diocletian also took giant steps to improve the economy, but his attempts to control inflation by mandate (prices were not allowed to rise – by order) proved to be a failure despite the most stringent penalties.

On Rome’s eastern front, Diocletian established numerous fortresses to watch over the Persians. The Strata Diocletiana provided a fortified main Roman road connecting many of the strategic points on the eastern frontier, and ran from Sura on the river Euphrates to the caravan city of Damascus. The new Persian king Narses (293–302) tried to renew hostilities against Rome with the invasion of Mesopotamia and Armenia in 296, followed by an incursion into Syria. The nearest tetrarch, Galerius, lost a battle in 297, but the new forts held firm and Galerius defeated Narses heavily in the following year, seizing the Persian ruler’s family and harem. Diocletian was able to dictate terms to Narses, resulting in a pact in 299 that ended some fifty years of hostility between the two kingdoms and enabled trade to resume. The peace lasted for forty years.

After twenty years of rule (305) Diocletian committed the unique act of resigning as emperor, and forced his reluctant colleague to do the same, so that the caesars now stepped into power and appointed in turn their successors. Unfortunately, squabbles began as to the appointments, another dreary cycle of civil wars began and finally the empire was reunited (324) under the sole reign of Constantine, who was joint or sole emperor from 306 to 337.

The individual rule of Constantine ushered in a new era, described variously as ‘the Police State’, ‘the beginning of the Middle Ages’ or ‘the end of the Roman Empire’. At first the new emperor was an ardent adherent to the Sun god, and Sol Invictus appears on his coins as late as 318. However, Constantine attributed his military victories in the civil war of 312 to a vision of Christianity, made it the official religion and received the title ‘The Great’, bestowed by a grateful Church. He created what we would today recognize as a medieval court, which he moved from Rome to a new capital on the Bosporus that he named modestly ‘Constantinople’. The new capital was furnished by removing valuables from other parts of the empire, particularly from pagan temples, and it possessed its own senate so that the ancient centre of power in Rome was greatly diminished. At the same time, all power was concentrated completely into the hands of the emperor, who served as head of state and head of the Church. Constantine was a king in all but name.

Because Christianity was now the official Roman religion, Constantine was in the happy position of being able to grab gold from the richly ornamented pagan temples in order to institute another revision of the currency. He introduced the new solidus, a gold coin minted at seventy-two to a pound weight of gold, and this high-quality coin would be retained as the standard gold issue for centuries to come – well into the Middle Ages. He also minted a new, high-quality silver coin at ninety-six to the pound weight. The follis continued to deteriorate and its production had largely ended by 353. Simple silver-less coins of base metals still provided the small change for day-to-day transactions.

Constantine extended Diocletian’s laws creating hereditary classes of citizens so that members could not move into any of the (many) occupations, such as the clergy, that were exempt from taxes. They could not even join the army. These changes resulted in widespread, overt hostility to the rule of the State. He also formalized the distinction between the frontier army and the better-paid mobile army, a decision that Zosimus would claim in the next century to have been responsible ultimately for the collapse of the Roman Empire. Worse, increasing use was made of barbarians within the army at all levels up to the top officers, while higher levels of immigration were permitted. The size of the legion, which had remained fairly constant at some 5,000–6,000 men for centuries, was reduced to 1,000 infantry and/or cavalry for flexibility of deployment. There were accordingly far more legions than previously.

The relative freedom from external enemies and civil wars allowed a final flourishing of Roman art in the middle of the fourth century. For a while pagan and Christian cultures coexisted comfortably in a nominally Christian empire, and this was when several historical works and the illustrated ‘Calendar of 354’ were produced. The latter shows that the birthdays of several of the ‘good’ emperors were still celebrated officially as public holidays, with circus races in their honour. Present in the list are the birthdays of Augustus, the first true emperor, Trajan, Hadrian, the Antonine emperors, Severus and, more recently, Claudius II Gothicus, Aurelian and Probus, followed by Constantine and his successors. Remarkable omissions from the feast days are the names of Diocletian, Carus and his sons, Valerian and Tacitus. Less remarkable are the omissions of the despised Commodus, Caracalla and Gallienus. The immediate successors of Augustus are also absent. The list of feast days includes the celebration of several pagan gods, including Sol Invictus, but lacks the celebration of the pagan god Mithras, still very popular but significantly never adopted by an emperor. Christian feast days are not yet commemorated. It was at about this time that the emperor Constantius II transferred the celebration of Christmas to 25 December, allegedly in order to counter the worship of the Sun.

There was a brief pagan revival under the last non-Christian emperor Julian (361–363) who, wisely, contented himself with encouraging pagan rituals rather than active persecution of Christians. While still a general, Julian had crossed the Rhine in 359 to harass the Alamanni after repelling their earlier invasion. In the east, the Persians again became aggressive under the rule of a new Shapur II. In the years from 337 to 350 Shapur made three raids into Mesopotamia and in 359 the Persians made a full-scale invasion, seizing several of the key fortress cities. Julian moved his armies to the east, but died in action; his Christian successor Jovian abandoned the Roman conquests so hard-won by Galerius in 298.

Again the empire had to be divided for defence, and the last strong emperor of the western empire, Valentinian (364–375), not only repelled a barbarian invasion but decided on a policy that had not been seen for decades. He carried the war back into the barbarians’ territory across the Rhine and ravaged their lands intermittently for the next seven years. Undoubtedly this strategy was made possible by the fact that Valentinian had appointed his loyal younger brother, Valens, as emperor in the east, so that Valentinian could deploy safely the great part of his army for reprisals. Valentinian seems to have impressed Jerome, who described him thus: ‘Valentinian in another time would have been an exceptional emperor, and was similar to Aurelian in his behaviour, except that his undue strictness and a certain frugality were interpreted as cruelty and greed.’ Valentinian died of apoplexy at the height of his triumph over the German tribesmen, and in the east Valens and his legions were overrun by the resurgent Goths – who had been allowed to settle within the Roman frontier – at the battle of Adrianople in 378. The army was almost wiped out after faulty tactics (probably part of the Roman cavalry engaged the enemy before the complete army was properly deployed) and then smoke from blazing fires blowing into Roman faces, and Valens was never seen again.

The victorious barbarians swarmed over Thrace but were unable to break into its walled cities. The new emperors of west and east, respectively Gratian and Theodosius, settled the Goths within the Roman frontier and recruited heavily from them to replace the vanished legions. Yet the barbarians served under their own chieftains and could be persuaded to adopt Roman military tactics only with difficulty, while they were also reluctant to wear their heavy Roman armour. Even the famous Roman curved, rectangular shield gave way to a lighter, circular type. The imposition of strict discipline on Roman armies had become troublesome from the early third century, with their propensity to make and unmake emperors; now it would be well-nigh impossible. Although neither the bravery nor, surprisingly, the loyalty to Rome of the new recruits could be criticized, the fact remains that by the end of the fourth century the ‘Roman’ army amounted largely to just another barbarian force. Less surprisingly, the new army was not successful against the numerically superior waves of other barbarians pouring in across the frontiers.

As the situation deteriorated, the increasingly Christian rulers and leaders of the empire took firmer steps to prevent its superstitious population from reverting to pagan worship in the hope of averting the barbarian onslaught. The use of public funds to pay for pagan ceremonies was halted in 384. In 389 all pagan festivals were stopped, excepting those deemed to be innocuous, such as the celebration of Roma Aeterna. Pagan temples were to be preserved as ‘ornaments’. Six years later, all pagan holidays were removed from the calendar but the games that once celebrated them were allowed to continue. The gladiatorial schools were closed in 399 and the last gladiatorial games were held in 404, being then replaced by wild beast hunts. The barbarian army, however, remained predominantly pagan.