Commodus,​ the only surviving son of the venerable Marcus Aurelius, lurched into megalomaniac excess soon after his succession. He thought he was divine, an incarnation of Hercules, and proclaimed imperial victories over Amazons and other imaginary peoples. He also fancied himself a gladiator (Ridley Scott’s film got that bit right) and delighted in slaughtering exotic creatures in the arena. Once, having decapitated an ostrich with specially designed arrows, he harangued the senators in their box seats, waving the bird’s head at them and declaring a fervent desire to do the same thing to the Senate. The historian Cassius Dio, then a young man, was there and recalls that he and his senatorial peers were torn between laughter and terror, knowing the emperor happily murdered those who displeased him. Eventually, after 11 years of burgeoning madness, Commodus was himself murdered.

The prefect of the praetorian cohorts, ten thousand elite troops stationed in a large camp at Rome (‘Praetorian Guard’ is a modern phrase, though a useful one), hatched the assassination plot in league with the emperor’s concubine and his chamberlain. They aimed to replace Commodus with a senator called Pertinax, potentially a voice of sanity and a man who would show his peers honour and preference in the same measure that Commodus had showed them disdain. On New Year’s Eve in 192, while the prefect entertained Pertinax at the Guard’s camp, Commodus’ concubine poisoned him. The drug knocked the emperor out but didn’t kill him, so his wrestling trainer was brought in to finish the job by strangling him as he vomited. By the next morning, Pertinax had been proclaimed emperor. The praetorians, at first reluctant to believe Commodus was dead, agreed to support the new emperor after being promised a large donative (an imperial accession gift, functionally a bribe). A joyful Senate convened to celebrate Pertinax’s accession and to denounce the mad god, now dead. Pertinax stopped them from having Commodus’ corpse ‘dragged on a hook’, but that was the limit of his good judgment. Commodus had bankrupted the treasury, so Pertinax imposed austerity on the palace, selling off courtiers’ luxuries to pay the guard its donative – a smaller one than he had promised. Mutiny and riot followed, until the Guard’s prefect gave up on Pertinax as well.

On the 87th day of his reign, Pertinax was confronted by hundreds of mutinous guardsmen and hacked to death. There was no organised plot and the killers had no plan. With no successor at the ready, the guardsmen auctioned off the throne to the highest bidder. Different praetorian cohorts threw their support behind different candidates, and when one of these, Didius Julianus, promised a lavish 25,000 sesterces per guardsman, they acclaimed him emperor and imposed him on a cowed Senate. But the marshals in the provinces would not accept Julianus and civil war broke out. The eventual victor, Septimius Severus, tricked the praetorian cohorts into greeting him outside the city walls in full parade uniform, but unarmed. Surrounding them with his own loyal troops, he stripped them of their uniforms, dismissed them from service and forbade them to come within a hundred miles of Rome on pain of death. Severus then constituted a new, hand-picked Guard from the best of the troops who had followed him. The auction story is impossible to resist and Guy de la Bédoyère duly starts with it. Similarly dramatic scenes pervade his book: whenever the praetorians enter the light of history, drama is unavoidable.

The Praetorian Guard was important because of its proximity to the emperor; in dicey circumstances, the guardsmen could make or break a reign, and often did. But there are huge gaps in what we know about them, whole decades in which we have next to no evidence concerning them whatsoever. The Guard originated in the late Republic, when rival dynasts employed hundreds, at times thousands, of experienced troops as bodyguards and symbols of their own power and status. Whether they were veterans recalled to the ranks or serving legionaries promoted on merit, they had in common a primary loyalty to their commander rather than to the Roman state, and a capacity to serve as tactical strike forces alongside whatever regular troops a general commanded. Large numbers of these praetorian cohorts were mustered and disbanded, or wiped out in the field, in the battles that brought the Republic down. In 30 BC, Julius Caesar’s grand-nephew Octavian, having eliminated every one of his rivals, inherited an exhausted world gasping for peace. He also inherited hundreds of thousands of men still under arms, not least his own and Mark Antony’s praetorian cohorts. He ended up discharging more legionaries than he kept in service, but the praetorians were another matter. Caesar had been assassinated for want of a bodyguard, and Augustus (as Octavian became in 27 BC) was determined to avoid the same fate. Though he played at being first among his senatorial equals, he was an autocrat who depended on a monopoly of force. Ten cohorts (probably) of a thousand men each (probably) were stationed in Rome’s suburbs, rotating into the city itself three cohorts at a time. These guardsmen served for less time than legionaries (at first 12 and 16 years respectively, later 16 and 20), and for greater pay: double or perhaps three times as much. Insofar as loyalty could be bought, Augustus bought it. The Guard was never commanded by a senator, whose loyalty could not be guaranteed, but by an equestrian (a lesser grade of aristocrat) who reported directly to the emperor.

As a mechanism for controlling the Senate and the Roman mob, the Guard was an efficient engine of autocracy. Yet as autocrats so often find, whatever is critical to the maintenance of power easily takes control of what it is meant to serve. Though the turbulence of the guardsmen was itself a major risk, it was their commander, the praetorian prefect, who posed the greater danger. Laetus, the prefect who killed Commodus, was far from being the most powerful of his kind. That was Lucius Aelius Sejanus (20 BC-AD 31), the prefect of Augustus’ successor Tiberius, who called Sejanus his socius laborum, ‘companion in toil’. Embittered and introvert, Tiberius was both an effective despot and a reluctant one, content to allow Sejanus as much power as he cared to acquire. Sejanus increased the size of the Guard and installed all 12 cohorts in a massive camp at the edge of Rome. Sejanus’ ambition became proverbial: Tacitus alleges that he had Tiberius’ son and heir poisoned; he certainly encouraged the emperor to retire from Rome to Capri and leave the business of government to him. For the better part of a decade, Sejanus was effectively emperor in Rome, exiling or executing inconvenient senators, not to mention members of the imperial dynasty whose claim to power might have been greater than his. The ancient sources contradict one another on the details of his downfall, but it is immortalised in Robert Graves’s I, Claudius in which Sejanus (a young Patrick Stewart in the BBC adaption) is presented by his successor, Macro, with the letter ordering his summary execution and the butchery of all his family. After seven years at the apex of power, Macro was driven to suicide by Tiberius’ young heir, Caligula.

Wherever there is good narrative evidence – Tacitus, primarily, but Cassius Dio and lesser lights as well – one finds a Guard prefect busily defending or undermining his emperor. Under Nero, the prefect Burrus and the philosopher Seneca rivalled the emperor’s mother, Agrippina, for power at court. But when Nero had his mother killed, Burrus was surprised to find his own influence diminished rather than strengthened. His death (probably from poison) put another strongman at the head of the praetorian cohorts: Tigellinus, whose desertion finally convinced Nero his reign was untenable and compelled him to kill himself. Timely desertion was a skill that would-be praetorian prefects had to master right up until the cohorts were dissolved for good. The emperor Constantius had abandoned his imperial patron Carinus before the crucial battle of the Margus in 285, thereby handing the empire to Diocletian and winning himself both the praetorian prefecture and an imperial bride. He would, in due course, become emperor, as would his son Constantine, though only after a flurry of complicated civil wars, in one of which the Praetorian Guard fought its last battle.

By 312, the empire had been governed by four legitimate emperors simultaneously (if not always harmoniously) for the better part of two decades, and each emperor had his own praetorian cohorts. How many of them remained in Rome that year is unclear, but they supported Constantine’s rival for power, the self-proclaimed emperor Maxentius, who ruled there. Crossing the Alps at speed in late summer, Constantine shattered Maxentius’ field armies in the north Italian plain, then marched on his outnumbered garrison at Rome. (De la Bédoyère gets that detail wrong, claiming that Maxentius’ forces were much larger than Constantine’s.) On 28 October, at the battle of the Milvian Bridge, Constantine’s seasoned troops massacred the praetorians and whatever other makeshift forces Maxentius had to his name. The defeated emperor drowned in the Tiber along with hundreds of guardsmen – a scene gloatingly depicted on the Arch of Constantine in the Piazza del Colosseo. The remnant was disbanded, the internal walls of the Castra Praetoria were dismantled and those of its gates that led through the city wall were blocked up. The office of praetorian prefect survived, but its incumbents became senior civilian administrators, never again to command troops, in battle or in rebellion.

De la Bédoyère tells these and other stories engagingly, and for the most part accurately. Only once does he really miss a trick, devoting two hundred pages to the first century of empire but skimming in two meagre pages the extraordinary relationship between Septimius Severus and his prefect Fulvius Plautianus, as steely a pair of terrorists as ever seized control of the empire. They came from Tripolitania, now the western half of Libya, a rough sort of place where the peasants still spoke Punic and Roman citizenship was rare. Neither man lost his regional accent or his preference for life in the provinces as they made their way at Rome, Severus a mediocre senator, Plautianus a promising equestrian. Through several strokes of luck, and the patronage of the prefect Laetus, Severus found himself governing a massive army on the Danube when the guardsmen killed Pertinax and auctioned the purple. He lost no time in marching on Rome and confronting Julianus, before turning to eliminate rival claimants who headed the Syrian and British legions. While Severus pursued their foes, Plautianus kept Italy in check with an iron fist. Their regime was terrifying not for refined displays of cruelty, but for the implacable efficiency with which enemies were hunted down and killed – the confiscated properties of condemned senators became so numerous that a new bureau of government was created to administer them. Plautianus twice shared the consulship with the imperial family; his image appeared on coins with Severus’ younger son, and the elder one, Caracalla, married Plautianus’ daughter Plautilla. That the old comrades would eventually fall out seems inevitable, but there is something melancholy about how it happened. In the winter of 202-3, Severus made a sentimental and triumphant return to Leptis Magna, the hometown to which he and Plautianus had gifted a monumental townscape that has so far escaped the ministrations of Libyan jihadists. At Leptis, Severus found that the citizenry had set up statues of Plautianus alongside those of the imperial family, failing to recognise the distinction in status of these equals in power. The infuriated emperor had his prefect’s statues melted down, and though the two made a show of reconciliation, the damage was irreparable. The emperor’s son, who hated his father-in-law, Plautianus, as much as he loathed his wife, Plautilla, plotted the prefect’s downfall. Having suborned false testimony from some officers, Caracalla accused Plautianus of treason and, in Severus’ presence, tried personally to stab Plautianus to death. Severus restrained his son, but not out of feeling for Plautianus, who was executed on the spot and had his body pitched unceremoniously into the street outside the palace. There is pathos here, at least to the degree that vicious men can inspire it.

De la Bédoyère’s book, like this review, is overwhelmingly anecdotal. However crisp and sprightly the storyteller, the Praetorian Guard is not a very good prism through which to tell Rome’s imperial story. The cohorts’ institutional history can be laid out in paragraphs rather than pages (the Oxford Classical Dictionary contains all the non-specialist needs). The careers of individual guardsmen, preserved in quantity in inscriptions, are a source of great scholarly importance, but they are no one’s idea of a gripping read. For that, one needs narrative, but as narrative, the history of the Guard is a case study in our limitations. Where there is a robust literary record (the first century AD, the turn of the second to the third), the Guard appears to stand front and centre at the heart of events. Where the narrative record is sparse or non-existent (most of the second and third centuries), the Guard looks like just another background element in the structural functioning of empire.

De la Bédoyère shows that even popular histories can illustrate the complexity of imperial power – that we must look not only at the emperor and his family, but at the networks swirling around shadowy figures like Macro or Laetus, as well as fully drawn characters like Sejanus. Conveying complexity, without subjecting the reader to the dozens of triple-barreled Roman names that make up the finer grain of imperial politics, has long been de la Bédoyère’s métier, as has the easy charm with which good popular history distinguishes itself. But those virtues deserve a more amenable subject.