The troubles began in 235, when the emperor Alexander Severus was murdered by soldiers at the age of 27 after Roman legions were defeated in a campaign against Persia. As general after general squabbled over control of the empire, the frontiers were neglected and subjected to frequent raids by such Germanic tribes as the Goths, Vandals and Alamanni, and outright attacks from Persia. Finally, by 258, the attacks were coming from within. In that year, the vast Roman provinces of Gaul, Britain and Spain all broke off to form the so-called Gallic Empire, and two years later, the eastern provinces of Syria, Palestine and Egypt became independent as the Palmyrene Empire, with Parthian backing.
An invasion by a vast host of Goths was beaten back at the Battle of Naissus in 268. This victory was significant as the turning point of the crisis. Victories by the emperor Claudius II Gothicus over the next two years drove back the Alamanni and recovered Spain from the Gallic Empire. When Claudius died in 270 of the plague, Aurelian, who had commanded the cavalry at Naissus, succeeded him as emperor and continued the restoration of the empire.
Aurelian ended the crisis during his reign (270-275) by hammering, in succession, the Vandals, Visigoths, Palmyrenes (see Queen Zenobia), Persians, and then the remainder of the Gallic Empire. By late 274, the Roman Empire was reunited and the frontier troops back in place. More than a century would pass before Rome again lost the upper hand on its enemies.
See these articles for more information on the leading figures of this time period: