Theodosius I

Theodosius I, known as Theodosius the Great, ruled from 379 until his death in 395 and was the last man to rule both halves of the Roman Empire. A general from Hispania, he took power after a catastrophic defeat by the Goths, made Nicene Christianity the official orthodoxy, and left the empire divided between his two sons.

He was born on 11 January, most likely in 347, in the Spanish provinces of the empire. His father, also named Theodosius, was a senior general under the emperor Valentinian I. The young man accompanied his father to Britain in 368–369 and by 374 commanded troops on the Danube, where he defeated a Sarmatian incursion. Soon afterwards his father fell from favour and was executed, for reasons that remain obscure, and Theodosius withdrew to his estates in Spain. Within a couple of years his family was rehabilitated and he was back in command.

In August 378 the eastern emperor Valens was killed by the Goths at the Battle of Adrianople, along with much of his army. Theodosius was chosen as his replacement and proclaimed emperor at Sirmium on 19 January 379, with the reluctant approval of the western emperor Gratian. He conscripted farmers and miners and enlisted many non-Romans, but could not expel the Goths. A treaty of 3 October 382 gave them land south of the Danube as allies who kept their own leaders, an arrangement that troubled later emperors. In the east he made peace with the Sasanian Empire, and the two powers divided the kingdom of Armenia between them in the late 380s.

After falling seriously ill in 380, Theodosius was baptised. On 28 February 380 he and his co-emperors issued the Edict of Thessalonica, which defined as "catholic" only those who accepted the Nicene teaching that Father, Son and Holy Spirit share one substance. In 381 he summoned the First Council of Constantinople, which confirmed that doctrine and condemned Arianism. Towards traditional pagan cults he was more cautious than later Christian writers claimed: he repeated bans on sacrifice but at first left temples open, though zealots, including some of his own officials, destroyed several shrines. Stricter laws against paganism followed in 391 and 392, but historians debate how far they were applied.

In 383 the usurper Magnus Maximus overthrew Gratian. When Maximus invaded Italy in 387, the young western emperor Valentinian II fled east, and Theodosius married his sister Galla. In 388 Theodosius defeated Maximus, who was executed on 28 August. Probably in April 390, after a riot in which a Roman general was killed, troops massacred thousands of people at Thessalonica. Scholars disagree about whether Theodosius ordered the killings. Ambrose, bishop of Milan, required him to do public penance before readmitting him to communion; the famous scene of Ambrose barring the emperor at the church door is now regarded as later legend.

Valentinian II was found hanged in May 392, and the general Arbogast raised Eugenius to the western throne. Theodosius marched west with an army that included Gothic allies and won the Battle of the Frigidus on 5–6 September 394, helped by a violent wind that blew against the enemy. He was now master of the whole empire, but he died of illness at Milan on 17 January 395. His sons Arcadius and Honorius succeeded him in the east and west respectively, and the empire was never again ruled by a single emperor.

Our 400 map shows the empire just after his death, split between the Western Roman Empire and the eastern court at Constantinople that would become the Byzantine Empire.