Nib asked
Why did the Roman Empire split?
Rome was never cut in two on one day: one empire kept being shared between rulers because it was too big for one, and the sharing after Theodosius died in 395 stuck for good.
7 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1too big for one
People picture Rome snapped in half on one day. It wasn't. In 286 Diocletian shared the throne because one ruler could not hold so vast an empire. In 293 he made it four.
2a second capital
Constantine ruled it all alone from 324. Then in 330 he dedicated a second capital, Constantinople, on the old Greek city of Byzantium, safe in the east.
3395
The last ruler of the whole empire was Theodosius, who won in 394. He died on January seventeenth, 395. His sons took the halves: Arcadius the east, Honorius the west.
4it stuck
Splitting had happened before and been undone. What was new in 395 was that nobody ever put it back together. The two courts drifted apart, each with its own armies and money.
5geography
The east held the better ground. Constantinople stood by a narrow strait, behind walls finished by 413, while the west had a long river frontier, the Rhine and the Danube, that was easy to cross.
6two fates
In 476 the last western emperor, Romulus Augustulus, was deposed by the German king Odoacer. The eastern half carried on for nearly a thousand years more, until 1453.
7the whole idea
So the whole story: one empire, too big for one ruler, shared again and again, and in 395 shared for the last time. Its two halves met very different ends.
Rome never split in an instant; it was shared until the sharing became permanent.