Late Roman Wales
The course of the later Empire (c 200-400) was a rocky one. Emperors came and went with bewildering frequency; sometimes several at a time.
The territory of modern Wales even had its own Emperors from 286-296, Carausius (Carawn in Welsh legend), and Allectus. The latter was defeated by Constantius, who ruled the Empire in the West, while Diocletian managed affairs in the East.
This split in the Empire was followed by a further split in the governing of Brittania, which was divided into four pieces. Although the divisions are not clear, it seems likely that Wales was part of Brittania Prima, which had its capital at Cirencester.
Throughout the second half of the 4th century, the Empire became increasingly unstable; barbarian attacks on the borders increased, and it seems that the legions were gradually withdrawn from Wales to counter threats on the continent.
By 390 AD there were probably no Roman troops remaining within the borders of Wales. In the next few decades, most of the legionaries in England followed and Brittania was essentially undefended.
The Irish saw their chance; in 405 pirates under Nial ravaged the western coast, and may have precipitated a fresh influx of Irish settlers.