A couple hundred years before it would fall for good, the Roman Empire faced a half-century of panic and defeat. Internal competition had split the once-great state into three conflicting portions, barbarian invaders ransacked the countryside, and a series of plagues depopulated much of the continent. Even the emperors weren’t safe, as one after another they succumbed to assassination, disease, and battle, the average span of rule during this period amounting to a mere two years. Even their nickname, the “barracks emperors,” betrays the speed with which they were hauled out, crowned, and used up.
Saving the empire came down to four men. Diocletian, when he realized that the task of administrating the empire was too much for one person to handle, split his authority first with co-emperor Maximian, then Constantius and Galerius as junior emperors. Of course they ended up feuding later on — they were still Roman statesmen, after all — but for the time being, Diocletian’s tetrarchy was enough to save the empire from total collapse.