The Third Century Crisis (dated abut 235 to 284 AD) was a time of trouble when many problems converged on the Roman Empire at the same time. Much of the empire’s northern frontier had always been unstable, but in the third century, the northern tribes were especially restless. Food shortages caused by climate change destroyed agriculture while raids from warlike tribesmen destroyed Roman forts and left settlements without protection.
As barbarian tribes grew more powerful, the startled Romans considered building a wall to fend off the invaders who were able to sack and destroy many important cities. Internal squabbles resulted in at least 27 different emperors holding power during the first half of the third century. By 260, it seemed that the empire would be destroyed by ineffective defenses. The western provinces of Gaul (modern France), Britain, and Iberia (Hispania/Spain) formed the Gallic Empire under the leadership of the usurper Postumus, who had protected them from invasion by German barbarians. After Postumus was assassinated by his soldiers in 268, the Gallic Empire lasted only 6 more years before being reabsorbed by the Roman Empire.
By the time Diocletian seized to power in 284 AD, “constant usurpations and rebellions” plagued the Empire. A usurper was almost always present in Britain throughout the third century, and turmoil continued in Gaul and Spain. Nevertheless, the legitimate Roman Emperors were able to maintain control of the British province until the Britons “expelled the Roman administration and began to manage their own affairs” in 409 AD. In 410, confronted by the Angles, Saxons, Picts, and Scotts, the Britons asked for help from then Roman Emperor Honorius. His refusal ended the era of Roman Britain. The Franks had complete control of Gaul (France) in 486, and Roman control of Iberia (Spain) was ceded to Persia in 363. The Iberians gained some autonomy from Persia by 406. By 476 AD the Western Roman Empire was defunct.