The Praetorian Guard — the bodyguard that sold the throne was abolished by the man it fought

The Praetorian Guard — the household troops created by the emperor Augustus around 27 BC to protect the person of the Roman ruler — became over three centuries the most dangerous force in Roman politics, and it was permanently abolished by the emperor Constantine in 312 after it had fought against him. On 28 October 312, Constantine defeated his rival Maxentius at the Battle of the Milvian Bridge outside Rome; Maxentius drowned in the Tiber, and the Praetorians who had made him emperor and died for him on the riverbank were the corps’ last stand. Constantine disbanded the survivors, scattered them to the frontiers, and demolished their great fortress, the Castra Praetoria, ending a body that had lasted some three hundred and thirty-nine years.

What fell was the original imperial bodyguard, and the institution that had turned guarding the emperor into the power to choose, depose, and even kill him. Augustus established the Guard as nine cohorts of elite soldiers, the only substantial armed force permitted near Rome, better paid and shorter-served than the legions. Concentrated by Tiberius into a single fortified camp on the edge of the city in 26 AD, the Praetorians acquired what no other unit possessed: a permanent armed presence at the center of power, alongside an emperor with no comparable force to balance them.

From that position the Guard learned that the men they protected served at their sufferance. They assassinated Caligula in 41 AD and installed Claudius; they intervened repeatedly in the succession; and in 193 AD, having murdered the emperor Pertinax, they put the empire up for auction and sold it to the highest bidder, Didius Julianus. They were a praetorian corps in the literal and the figurative sense — a bodyguard that had become a kingmaker, repeatedly trading their loyalty for donatives and treating the throne as a prize in their gift.

The mechanism of the fall was the corps backing the losing side in a civil war fought by an emperor strong enough to dispense with them. When the tetrarchic system fractured, the Praetorians in Rome proclaimed Maxentius and fought for him to the end. Constantine, commanding a battle-hardened field army of his own and owing the Guard nothing, beat that army at the Milvian Bridge and then abolished the corps outright rather than buy its loyalty as so many predecessors had. The reforms of the era had already made the old Guard militarily redundant; Constantine’s victory made its destruction final.