Forty Years of Civil War

Maximinus Thrax, despite being chosen by his men, was killed by them when they had to go without food while sieging Rome.

The next forty years or so would see Roman Generals fighting for the throne. Mostly Barracks Emperors. Many a Legion picked its own commander to put forward as an Emperor.

While they were fighting, foreigners invaded.

In the West, the Gauls broke away to form the Gallic Empire.

In the East, the Arabs broke away to form the Palmyrene Empire

The Emperor Valerian was captured by the Sassanids.

Rome could have fallen right then.

But it didn't.