Praetorian Guard
The emperor's elite bodyguard and the most powerful armed force in Rome, who over three centuries went from protecting rulers to assassinating them and auctioning the empire.
Augustus built the Praetorian Guard to solve a simple problem: a ruler needs soldiers near him, but Rome forbade armies inside the city. His answer was a corps of about 9,000 elite troops, nine cohorts, who drew far higher pay than the legions on the frontier, served a shorter term, wore the toga in the city like civilians, and were stationed in Rome itself, where armies were otherwise forbidden, though only three of the nine cohorts served there at a time, the rest billeted in nearby Italian towns. It was a pampered, privileged force, and it sat at the center of imperial power. For three centuries they guarded the person of the emperor. They also learned that the man closest to the throne is the man best placed to empty it.
Proximity becomes power
The turning point was physical. Under Tiberius, the ambitious prefect Sejanus gathered the scattered cohorts into a single fortified barracks, the Castra Praetoria, on the edge of the city. Now the Guard was concentrated, armed, and within a short march of the palace. In 41 AD they made the logic explicit: a group of officers murdered Caligula, then found his stammering uncle Claudius hiding behind a curtain and hailed him emperor. Claudius paid for the honor, 15,000 sesterces a man, and the precedent was set. An emperor now took the throne by buying the Guard's loyalty, and every successor understood the invoice would come due again.
The empire for sale
The system reached its logical bottom in 193 AD. The Guard murdered the emperor Pertinax, who had tried to discipline them, then announced from the walls of their camp that the throne would go to the highest bidder. Two rich men bid against each other; a senator named Didius Julianus won by promising 25,000 sesterces per soldier. He had, in the most literal sense, purchased the Roman Empire at auction. It bought him 66 days. The frontier legions refused to accept a ruler chosen this way, marched on Rome, and killed him. The auction is only the most flagrant example of a habit: over their history the Praetorians murdered, made, or extorted a long line of emperors, and no ruler could feel entirely safe with them at his back.
Guarding the guards
The Guard's story ends at the Milvian Bridge in 312 AD, where they fought for Maxentius against Constantine and lost. Constantine, no sentimentalist, disbanded them outright and demolished the Castra Praetoria. Their three centuries left a warning that outlived Rome: any force built to protect a ruler acquires the power to remove him, and proximity is its own kind of authority. The Ottoman sultans relearned it from their Janissaries, elite slave-soldiers who deposed sultans until Mahmud II destroyed them in 1826. Modern strongmen relearn it too, which is why presidential guards are watched as closely as any opposition. The word we still reach for, a praetorian faction, carries the whole lesson in a single adjective: the sword that stands behind the throne is always tempted to sit on it.