Why did the Roman Empire split into East and West?

Too many frontiers for one man

By the third century AD the empire had to hold the Rhine and the Danube against Germanic peoples and the Euphrates against a new and aggressive Persian dynasty, often in the same year. An emperor could be in only one place. Wherever he was not, the local army was tempted to proclaim its own general. In the half century after 235, emperor after emperor was murdered by his own troops.

Sharing the job was the obvious answer. In 253 the emperor Valerian left his son Gallienus in charge of the West and took the East himself. It ended badly. In 260 the Persians captured Valerian alive, a breakaway empire held Gaul until 274, and in the East Queen Zenobia of Palmyra for a time ruled Syria and Egypt.

Diocletian’s four rulers

Diocletian, a soldier from Dalmatia proclaimed in 284, made the sharing permanent. In 286 he named a fellow officer, Maximian, co-emperor, with the title Augustus, to run the West, and kept for himself the East, the richer half and the one facing Persia. In 293 each Augustus took a junior partner, a Caesar: Constantius in the West, Galerius in the East. Historians call it the tetrarchy, the rule of four. Each ruler had his own court, army and treasury near his own frontier. Rome kept its Senate and its prestige, but no emperor lived there.

The four insisted they were one government. Laws went out in all their names. A porphyry group of the tetrarchs, now set into a corner of St Mark’s Basilica in Venice, shows them in near-identical pairs, embracing. Venetians carried it off from Constantinople after the sack of 1204; a missing foot was later found in Istanbul.

One empire, two capitals

The system broke down in civil war, and Constantine had reunited everything by 324. But he also gave the East a capital of its own. Constantinople, dedicated in 330, got its own senate (why he built it). From then on, a western court and an eastern court were always possible. At Constantine’s death in 337 his sons shared the empire. In 364 the new emperor Valentinian took the West and gave the East to his brother Valens. Ammianus Marcellinus describes the two dividing the generals and the troops near Naissus, today’s Niš in Serbia, before one rode to Milan and the other to Constantinople.

The division of 395

Theodosius I was the last emperor to rule both halves in person, and only for a few months, after he crushed a western usurper at the river Frigidus in September 394. He died in Milan in January 395. His elder son, Arcadius, about seventeen, already reigned in Constantinople. The younger, Honorius, aged ten, took the West, under the guardianship of the general Stilicho. The line between them ran down the river Drina, still a border today, and across the sea to Libya, between Tripolitania and Cyrenaica. It roughly followed the old divide between a Latin-speaking West and a Greek-speaking East, though Latin was still heard in the eastern Balkans.

Two empires, or one?

One, as far as Romans were concerned: a single Roman Empire with two emperors. Laws still carried both names, and each half usually named one of the year’s two consuls. In 438 the Theodosian Code, a collection of imperial laws compiled in Constantinople, was formally received by the Senate in Rome. But the two courts became rivals, with their own armies, ministers and quarrels. When Odoacer retired the last emperor in Italy in 476, the Roman Senate sent the regalia east with a message, reported by the historian Malchus: one emperor was enough for both. On paper the empire was whole again. In fact the West was gone (why the East survived).