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why did the roman empire split
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.
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.
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.
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.
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.
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.
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.