The new emperor Diocletian was the son of enslaved people, with a mother from the province of Dalmatia. He appointed an army officer from modern-day Serbia, Maximian, as his co-emperor. In terms of the empire’s political system, Diocletian went even further. He decided there should be four emperors: two senior emperors with the title of Augustus and two junior emperors, the Caesars.

The two chosen Caesars were the generals Galerius and Constantius. The four emperors split the empire between themselves. Galerius had the provinces on the Danube in Eastern Europe, Constantius was responsible for Britain, Gaul, and Spain, Maximian held Italy and Africa, and Diocletian kept Greece, Egypt, and the Middle East.

Still, the empire struggled with revolts and outside attacks. Maximian had to crush a rebellion by the exploited peasants of Gaul. Diocletian fought attacks from groups in North Africa. Britain broke away from the empire under the rule of Carausius. By the time Constantius reconquered Britain, Carausius had been assassinated and replaced by his own advisor, Allectus.

In order to address these problems, Diocletian fortified the borders of the empire and adapted Probus’s policy of allowing “barbarian” tribes to settle depopulated areas.