The Soldier Emperors
The third century was one of appalling disaster for the Roman empire. The army realized its power, made and unmade emperors for cash and ceased to be an effective fighting force.
Barbarians invaded and plundered Roman territory. Pestilence, in addition to war, greatly diminished the population. It seemed as if the empire was about to fall. This result was averted by two energetic men, Diocletian and Constantine.