Explanation: During the 3rd century AD, the Roman Empire went through a prolonged crisis known as... well, the Crisis of the Third Century. Everything that could go wrong, essentially, did. Plague, invasions, secession, coups, civil wars, military rule, hyperinflation, corruption, banditry, breakdown of law, religious fanaticism, rising popularity of trousers...
The Empire survived, but only barely - and in a form hardly recognizable to the Principate of the first two centuries AD.