Rome divides and falls

By the year 300 CE, the Roman Empire was too big for one person to govern well. It stretched from Britain to Egypt, and a message from the capital could take weeks to reach the far edges. In 285 CE the emperor Diocletian split the empire into an eastern half and a western half, each with its own ruler, just to keep the government working. The two halves grew apart over the next century -- different languages (Latin in the west, Greek in the east), different economies, different problems.

The Western Roman Empire did not fall in a single battle. It weakened for over a century from several causes at once: emperors were assassinated or overthrown so often that the army stopped trusting the government, heavy taxes and a shrinking economy made it hard to pay soldiers, and the army increasingly relied on hired Germanic warriors whose loyalty was to their pay, not to Rome. Meanwhile, whole peoples -- Visigoths, Vandals, and others -- were pushed toward Roman land by the Huns migrating out of Central Asia, and Rome no longer had the strength to hold its borders. In 410 CE the Visigoth king Alaric sacked the city of Rome itself, a shock that had not happened in 800 years. The traditional end date is 476 CE, when the Germanic general Odoacer removed the last western emperor, a teenager named Romulus Augustulus, and did not bother replacing him.