In the period discussed here, the Crisis of the Third Century seemed like a thing of the distant past. The Tetrarchy functioned well and the four emperors of the Roman Empire won important victories on all fronts. Rebellions were crushed, rebel leaders were eliminated, and after a few initial setbacks even the archenemy of the Romans, the Sassanid Persian Empire, was brought to its knees. The Roman victory over the Persians led to a long-lasting peace, which was not broken until several decades later. The Roman Empire appeared to be heading towards a new Golden Age.
Constantius retakes Britannia
After capturing Gesoriacum (Boulogne) from Carausius in 293, the caesar Constantius probably started planning his invasion of Britannia immediately. The island was under the control of the rebel Allectus, who had murdered Carausius and was presumably only able remain in power thanks to the aid of a contingent of Frankish mercenaries. In 296 Constantius launched his long expected attack. He had formed two separate fleets and split his invasion force in two. The caesar took personal command of the main fleet, delegating command of the second fleet to Julius Asclepiodotus, Constantius’ praetorian prefect. Unfortunately bad weather forced most of Constantius’ fleet to return to base. Just a small part of his army managed to land in the south of Britannia. Asclepiodotus was more successful. All of his troops succeeded in landing safely, but then refused to advance because they wanted to wait for the army of Constantius. In response, Asclepiodotus had all the ships burned down, so that the soldiers had no option but to fight.
In the vicinity of Londinium (modern London) Asclepiodotus’ men clashed with Allectus’ army. In the battle Allectus was defeated and killed. His surviving mercenaries hastened back to Londinium, determined to pillage the city before making their escape. According to tradition they were stopped by the very few soldiers from Constantius’ army that had made it to Britannia. These now saved the city from a horrible fate. Constantius himself presumably did not arrive on the island until well after Asclepiodotus’ victory, but he was hailed as a great saviour nonetheless. Prisoners from the army of the defeated Allectus were massacred and about a year later an orator wrote a famous panegyric (panegyricus) about Constantius’ victory.
Victories over the Persians and in Egypt
Around 296 the Persian king Narses had started a new war with the Romans. He was the youngest son of the great Shapur, who in the past had ever so often left the Romans in the dust and had even taken their emperor Valerianus prisoner. Narses invaded Roman Mesopotamia and overran part of the region. Mesopotamia was the easternmost part of the diocese of Oriens, and since Diocletianus ruled over this part of the Roman Empire, it seemed likely that he would personally take charge of the war against the Persians. However, the augustus left the counteroffensive to his caesar Galerius. This may have been a reward for the successes that his adopted son and son-in-law had won along the Danube border. An alternative explanation is that Diocletianus was at the time busy fighting a certain Lucius Domitius Domitianus, who had rebelled against the emperor in Egypt around the year 297 and had occupied Alexandria. Alexandria was one of the largest and most important cities in the Empire, so Diocletianus had to set his priorities.
After arriving in Syria, Galerius crossed the river Euphrates and subsequently confronted Narses’ army somewhere between Callinicum (Raqqa) and Carrhae (Harran). According to Eutropius, his army was much smaller than that of the king, and the result was an ignominious Roman defeat. Galerius returned to Antioch, where he had a meeting with Diocletianus, who had just crushed the revolt in Egypt. Again according to Eutropius, the augustus humiliated his defeated caesar by having the man run alongside his chariot for several miles while still wearing his imperial purple robes (purpuratus). The story is possibly apocryphal, but Diocletianus must have been livid about the Roman defeat against the Sassanids. However, he decided to give Galerius a second chance. For the new campaign additional forces were called up from the Balkans, and the next year the caesar would again face Narses on the battlefield.
Now over to Diocletianus and his successful campaign in Egypt against Domitianus and his ally and probable successor Aurelius Achilleus. The rebels had entrenched themselves in Alexandria, which had then been besieged by Diocletianus for eight months. After taking the city, the emperor had gone on a killing spree. According to a rather absurd story the inhabitants of Alexandria were ultimately saved by Diocletianus’ horse. The emperor had ordered his men only to stop butchering their enemies when the blood in the streets came all the way up to the knees of his horse. As his horse tripped, the order was complied with much faster than expected. The grateful Alexandrians supposedly then erected a statue for the horse. The source of the story is John Malalas, a Syrian who wrote in the sixth century, long after the events. His story seems very implausible, but there can be little doubt that the emperor treated the Alexandrians very harshly. After a previous bloodbath instigated by Caracalla in 215, this was the second time in the third century that Alexandria had seen such ruthless violence.
Presumably in the spring of 298 Galerius invaded Armenia. The Roman army clearly favoured the mountainous terrain here over the flat plains of Mesopotamia. The caesar managed to catch Narses by surprise, possibly during a nigh attack, and heavily defeated the king and his army. The Romans captured the king’s camp and took his wives, children and various members of his royal household prisoner. Moreover, they confiscated the Persian treasury. In the meantime Diocletianus had also arrived with an army in Mesopotamia, and the two men met in Nisibis. From there Galerius advanced further along the Tigris. The campaign ended with the capture of Ctesiphon, the Persian capital. Among the men who conquered the city was a young officer named Constantine, the son of Constantius. The Roman victory allowed him to visit the ruins of Babylon, the famous ancient city that had once stood not far from Ctesiphon.
In 299 the Romans and Persians made a new peace treaty. The peace terms were highly favourable for Rome. Narses returned Roman territory in Mesopotamia and accepted Roman supremacy over a string of little kingdoms, including Armenia and Iberia, which were henceforth considered Roman protectorates. The Tigris became the border between Roman and Persian territory and Nisibis became a Roman city again. Only there were merchants allowed to cross the border between the two Empires, a measure intended to regulate trading contacts and facilitate tax collection. After the treaty had been signed, or perhaps even before that, Galerius returned to the Balkans, where he campaigned against the invading Marcomanni. The caesar subsequently dedicated himself to embellishing his ‘capital’ of Thessalonica in Macedonia, where among other things he had a palace built and a triumphal arch in honour of his victory over Narses.
Maximianus in North Africa, Constantius in Gaul
In 297 at the latest Maximianus had crossed over to North Africa to fight against a coalition of Berber tribes known as the Quinquegentiani. Some ten years ago these had invaded Roman territory, which had among other things led to the loss of the city of Volubilis in present-day Morocco. Although they had been expelled again by the Roman governors in the region, the tribes had launched fresh attacks several years later. It was therefore only proper that the augustus himself now came to the region. No details are known of Maximianus’ subsequent campaign, but it is likely that he dealt with the invaders ruthlessly. It was a tactic that worked, as the cities of Lixus and Tingis (Tanger) in Mauretania Tingitana, in the far west of North Africa, were retained for the Romans. They were then made part of the diocese of Hispaniae. Volubilis, on the other hand, was not retaken. It remained a culturally Roman city outside the Roman Empire until the coming of Islam several centuries later. Maximianus followed up on his success by reorganising and strengthening the southern border of the Empire from Numidia to Tripolitana. On 10 March 298 the emperor held his triumph in Carthage to celebrate a most successful campaign.
After his victory in Britannia, Constantius had not stayed on the island for long. He had quickly returned to Gaul, which at the time also included the former Germanic provinces. The caesar decided to settle in Augusta Treverorum (Trier), which he turned into his capital. In 298 the Alemanni broke through the Rhine borders. Constantius pursued them and bumped into their main force at Andematunnum (Langres), in the territory of the Lingones. Presumably the caesar had ridden ahead with just a small force, as according to Eutropius he suddenly found himself under attack and had to be hoisted onto the walls of the town with a rope. The tide fortunately turned quickly when his own army appeared on the scene. The Romans won a large victory, although the 60,000 dead Alemanni mentioned by Eutropius must obviously be taken with a pinch of salt.
Now that so many splendid victories had been won, it was high time for the Romans to construct beautiful monuments and other buildings. The Arch of Galerius in Thessalonica has already been mentioned. In Rome the Baths of Diocletianus were built on the Viminal hill between 298 and 306. They covered a terrain of some 380 by 370 metres, which made them the largest in the city. Although the complex was named after Diocletianus, the baths were in fact built on the orders of Maximianus, who was, after all, the emperor in control of Rome. Before the construction could start, a large number of buildings had to be demolished, but the Temple of the Gens Flavia, built by the emperor Domitianus (81-96), was not touched and became part of the complex. The Baths of Diocletianus were larger than those of Caracalla, which had opened their doors in 216. They could accommodate up to 3.000 people. Substantial parts of the baths have been preserved. These parts currently house two churches and a branch of the Museo Nazionale Romano.
Another important building in Rome that bears the mark of the Tetrarchs is the Senate building on the Forum Romanum, the Curia Julia. In 283, during the reign of Carus and his sons, the building had been heavily damaged by fire. Several years later the Tetrarchs had it mended. The project is usually attributed to Diocletianus, but as with the baths we should probably give credit to Maximianus. What is rather ironic about the restoration of the Senate house is that it was a purely symbolic project. Under the Dominate the Senate had become a marginal factor. The importance of Rome as caput mundi had also diminished. When not in Rome, the Tetrarchs usually resided in their own capitals, which were much closer to the borders of the Empire.
- Aurelius Victor, De Caesaribus 39 (translated and annotated by H.W. Bird);
- Eutropius, Breviarium Historiae Romanae 9.22-25.
- Adrian Goldsworthy, The Fall of the West, p. 161-162 and p. 172-173;
- Henk Singor, Constantijn, p. 147 and p. 298;
- Timothy Venning, A Chronology of the Roman Empire, p. 631-634.
 Eutropius, Breviarium Historiae Romanae 9.24.
 Adrian Goldsworthy, The Fall of the West, p. 173; according to Timothy Venning, A Chronology of the Roman Empire, p. 633, Narses rejected the clause about the status of Nisibis. Here I follow Goldsworthy.
 Eutropius, Breviarium Historiae Romanae 9.23.
 Diocletianus probably visited Rome only once, in 303, for the celebration of his Vicennalia, his twentieth year on the throne.