The troubled third century
Caracalla reign ushered in a terrible period for the Roman Empire, with emperor following emperor in quick succession, many lucky to last a couple of years before being murdered or killed in battle by their successor. The empire itself was under constant attack from the barbarians and enemies with many areas devastated by war. The unfortified cities on the continent were to prove easy picking for the attackers. To add to the misery plague ravished the empire, killing many people including the Emperor Claudius II Gothicus who had actually managed to defeat the Goths. It wasn't until the reigns of Aurelian and latter Probus that the situation started to be brought back under control.
One part of the empire that managed to avoid the disasters of the third century was Britain. There seems to have been few raids on the province, with many of the garrisons along the wall run down in size, and some of the forts allowed to fall into a state of disrepair. It very likely that some of the commanders fell into the practice of claiming pay for soldiers who had died or didn't exist, a corrupt practice common throughout the Empire at this time.
Postumus and the Gallic Empire.
This prosperity may have been partly due to the man who ran Britain, because for over a decade Britain was not ruled by the Emperor in Rome, but by Postumus and the Gallic empire. Postumus came from a humble background, but was an extremely gifted soldier. He was appointed by the Emperor Valerian to command the Rhine legions. While in charge of these he rebelled against the Emperor Gallienus and took over Britain, Spain and Gaul. He ruled wisely for a decade and succeeded in defeating all the attacks from the German tribes, as well as all efforts by the Emperor to regain these provinces. He was eventually murdered by his own troops after he refused to allow them to sack Mainz. Postumus was followed by a quick succession of leaders before the Gallic Empire was regained by the Emperor Aurelian.
Britain hadn't finished with rebellion however, and in AD287 the remarkable figure of Carausius appeared. Carausius had been appointed as commander of the Roman fleet at Boulogne; with orders to destroy the Frankish and Saxon pirates. However by some accounts he was in league with them. To escape the Emperor's anger he sailed to Britain, defeated the governorís troops and took possession of the place. All attempts to drive him out failed and he even managed to extend his hold over parts of Gaul. In 293AD Constantius took up the fight with Carausius and soon managed to drive him off the continent. However it wasn't until Carausius had been murdered by his chief minister Allectus; that Constantius managed to invade and recover the Island.