Walk along the Aurelian Walls and you are walking along the moment Rome began to feel afraid.
Built between AD 271 and 275, they were the first walls to surround the entire city in centuries.
The fact that they were needed tells you everything.
For the first time in generations, Rome could no longer assume the empire would protect it.
The third century was a catastrophe.
Between 235 and 284, Rome had 26 emperors in 50 years.
Most of them were murdered by their own armies.
Plague swept the empire.
Currency collapsed.
Frontiers buckled.
Diocletian stabilised things temporarily by splitting the empire in two.
Constantine reunified it, moved the capital to Constantinople, and made Christianity the official religion — transforming Rome's identity forever.
But the western half continued to weaken.
In 410, the Visigoths sacked Rome — the first time the city had fallen to an enemy in 800 years.
The shock reverberated across the entire world.
In 476, the last western emperor, a teenager named Romulus Augustulus — a name of cruel irony — was deposed.
The empire in the west was over.
Fun fact: Edward Gibbon's 'The Decline and Fall of the Roman Empire', published in 1776, runs to six volumes and 1.5 million words.
Historians have been arguing about the causes ever since.
The current count of proposed explanations is over 200.
