Severus was another ambitious emperor, and he stayed around for awhile, leading a campaign into Scotland that some sources say numbered in the tens of thousands.
They met yet again with a primitive form of guerrilla resistance, and the whole thing bogged down when Severus died of illness in 211.
The result of this division was relative quiet, called by some historians the Long Peace. The Romans and Britons built a series of forts along the southeast coast, nominally to protect against piracy. These became the Saxon Shore forts.
Near the end of the 3rd Century, Britain was divided into four provinces. Administration continued as before.
The 4th Century was punctuated by continual low-level unrest. One high-level disruption in 367 was the famed Barbarian Conspiracy, in which troops along Hadrian’s Wall rebelled and allowed entrance to Caledonian Picts. It took a determined Count Theodosius at the head of a determined relief force to settle the matter. But Rome’s ability to settle the matter was becoming increasingly weak.
At the same time, Roman troops were under attack elsewhere in the Empire, in Gaul and in Africa and along the Danube frontier.
The term "barbarian conspiracy" comes from the Roman historian Ammianus Marcellinus. Despite the name, historians don’t have any evidence to suggest that it was a coordinated attack. It could have been merely coincidence. But even a military as mighty as Rome’s had trouble fighting off that many different attacks in that many different parts of the world.
As the 4th Century ended, more and more Roman troops were ordered away from Britain and back to Rome proper, to deal with increasing barbarian incursions. Constantine III took most of the remaining troops with him when he crossed the Channel in 407 and set himself up as a usurper Western Roman Emperor. Constantine didn’t last long in that position, seeing his reign and his life end at the hands of the proper emperor, Honorius.
It was that emperor who issued a final refusal of Roman help to Britain, in 410. Roman troops would not be coming back.
The Roman influence was far from gone, however. They left behind a strong, widespread legacy.
They left behind roads and bridges and aqueducts and baths and temples and buildings galore.
They left behind their laws and their social structures and their orderly way of life.
They left behind Christianity. Much of Roman attention had been given to reducing the power of the Celtic Druids, early on not so much on religious grounds as for political purposes.
More than a decade before, however, Severus had divided Roman Britain into two parts: Britannia Inferior, the northern part, had its capital at Eboracum, what is now York. Britannia Superior, the southern part, had its capital at Londinium, what is now London. Historians are not exactly sure where the dividing line was, although some maps do their best to approximate one.
