Constantine the Great
After his successful campaign in Scotland, Constantius I returned to York and died their in 306AD. His son Constantine had accompanied him on the campaign, and on the death of his father was proclaimed Emperor (encouraged by the charmingly named King Crocus who has in command of some of Constantines German troops. This was in clear breach of Diocletian's new constitution, with emperor's for the eastern and western Roman empire both supported by a Caesar. There followed a long and complicated series of bloody civil wars, Constantine only finally emerging as sole emperor of a united empire in 324 AD.
Constantine liked to make much of the way he had come from the far West of the empire, swept as if by a divine wind across the entire empire. He showed great military and political abilities (not to mention ruthlessness) to win supreme power and brought in huge changes. He became the first Christian emperor, believing that his major victory at the Milvian bridge was due to the Christian God he ended the persecution of the Christians. After Constantine only one emperor (Julian) would attempt to bring back the old Roman Gods. Constantine finally died while planning a campaign against the Persians at Nicomedia in 337 AD.
Constantine the Great huge personality and prestige held the empire together and led to an age of peace and prosperity in Britain, but this was soon to unravel. After his death there was no successor at all for the first four months, but finally the emire was split between his three sons, Constantine II who was in charge of Britain, Gaul and Spain, Constans with Italy and North Africa and Constantius II in charge of Constantinople and the East (unfortunately Constantine displayed a great lack of immagination with their, which made the events that followed even more confusing!). The arrangement didn't work with first Constantine II invading Italy.
Constantine II invasion was a disaster and he was defeated and killed at Aquleia by Constans army. It must have come as a nasty shock to Roman Britain, but the effect on the Roman Army is unknown; although it has been suggested that it was severely weakened. What is certain though, is that the emperor Constans himseld visited the Island in the middle of Winter of 342/343 AD. Whatever brought Constans to britain must have been important, for it was extremely unusual to cross the Channel at that time - however the reason is as likely to have been to crush internal dissent as to deal with an external foe.