Now, what to say about the politics from Constantine to Honorius and Arcadius? (Since we already discussed Diocletian with the third century.) For the first portion of Constantine’s reign, there were civil wars until 312 when he defeated his western rival Maxentius at the Milvian Bridge (it was around this time that Constantine had his conversion to the god of the Christians), thus becoming sole emperor of the West. In 324, he defeated his eastern colleague Licinius (allegedly over persecution of Christians) and become sole emperor of the whole Roman world.
Let us remember that the whole Roman world, the empire, at this time stretched from Britain to Spain and North Africa, across to Mesopotamia and down the Nile. There were Black Sea provinces. There was influence in Armenia. Despite the images people like to paint of Roman hegemony after the 200s, the recovery of Diocletian and Constantine meant that Rome maintained and stabilised her territorial possessions.
Constantine had spent much of his earlier career based in Trier on the Moselle (a river which immortalised by Ausonius in this poem). In 330, he rededicated Byzantium on the Bosporus in the East as Constantinople — destined to be the New Rome and a Christian capital. Upon his death in 337, there was a bit of a bloodbath (see R W Burgess, ‘The Summer of Blood’). His son Constans ruled from the West until 350, and Constantius II ruled the East, then the whole thing, until 361. Constantius, as you learn in Ammianus, had a few bloodthirsty caesars co-rule with him. He was succeeded by the ‘pagan’ Julian the Apostate (would have called himself a Hellene), himself an orator, poet, and philosopher, who did to the Christians what Constantine and Constantius had done to the Graeco-Roman polytheists. He died on campaign against Persia in 363 (see my post on Julian in Ammianus).
Then came Jovian, hailed by the army in Persia. He lasted nary a year and was succeeded by Valentinian I (r. 364-375) who elevated his brother Valens as co-emperor (r. 364-378). Valentinian’s son Gratian (d. 383) was elevated in 367 and Valentinian II (d. 392) upon Valentinian I’s death in 375. 378-395 saw the reign of Theodosius I ‘the Great’.
Upon Theodosius’ death, the empire was finally and enduringly split between East and West, his son Honorius taking the West (r. 393-423) and his other son Arcadius the East (r. 395-408).
Those, in brief, are the emperors of the fourth century.
Throughout these reigns there was ongoing warfare and military posting along the Rhine. As well, Goths were interested in crossing the Danube. They were settled on Roman territory in 376, but, rather than treat them well and integrate them into society, relations between the Romans and Goths deteriorated to the point where the Goths decided it was in their best interests first to plunder, and then to engage the Romans in battle. The most famous of these battles was near Adrianople in Thrace (Edirne, modern European Turkey, near Greece & Bulgaria) in 378, where the Emperor Valens was slain.
War with Persia was also intermittent. As I mentioned in the post about the third century, Persia had re-emerged as a major power in the East under the Sassanian dynasty, taking the Emperor Valerian captive in 260. Despite a treaty made in 363 after Julian’s disastrous campaign, the Persians found pretext to invade Rome, claiming the Romans had violated the treaty. Before I start sounding like a Roman talking of ‘typical’ Persian ‘duplicity’, I can well imagine the Romans having broken the treaty if it had been in their interest and they had had a pretext. Theodosius I and Shapur III signed a peace treaty in 384 that would last until 421.
It wouldn’t be the Roman Empire without a few usurpers, either, though. The most famous of these is Magnus Maximus, a Spanish general posted in Britain who rebelled against the Emperor Gratian in 383. (Sorry that he’s not actually British.) In 384, Theodosius I agreed to give him command of Britain. However, in 387 he wanted more than he already had, so he invaded Italy. In 388, Theodosius defeated him at the Battle of the Save. I would say that Maximus was more tolerated than fully integrated into the imperial system. Northern Gaul was often a troublesome place, and local aristocrats had previously, and would again in the future, taken the power of command into their own hands.
I feel that this has been far too cursory a treatment of fourth-century politics, but I also don’t want to go on too much. My apologies for this. I hope, however, when combined with the prior post on religion and literature and the upcoming post on art, it will help give you a feel for the fourth century and the people who inhabited it.