Pax, the latest installation in Tom Holland’s trilogy on the rise and fall and rise again of the Roman Empire, closes with this striking vignette: Publius Aelius Traianus Hadrianus Augustus, the most powerful man on the face of the earth, arrives at the furthest reach of his dominion – the River Tyne. Hadrian had travelled to Newcastle to do something extraordinary: to inaugurate a new era of Roman power, and a new conception of the empire. By instructing his legionaries to run a brick wall through Byker, Hadrian gave the empire something it had never had before – a formal limit. Along with his palisade on the Rhine and his ditch in Algeria, Hadrian’s Wall was both a message and a warning: what is good and what is valuable in this world is to be found on my side of the fence; what is meagre and what is lowly is without. The empire of Hadrian and his Antonine successors (the time and place which Gibbon proposed as the happiest and most prosperous for the human race), was to become a walled garden – within its limits the fruits and blossoms of civilisation would be tended and nourished. The selfish giant would repose in his capacious armchair and squish any nasty little children who tried to clamber in to sample a peace that was not rightly theirs to enjoy.

The issue of limits and their transgression transports us back to the image with which Holland began this story two decades ago in Rubicon, his acclaimed and best-selling history of the Roman Republic, which opened with another powerful Roman general standing on the banks of a northerly river. Combined with Dynasty, his history of the Julio-Claudian emperors, we can now read a continuous account of how Julius Caesar’s decision to cast the die and cross the Rubicon into Italy with his outlawed legions brought the aristocracy of the Roman Republic crashing to the ground and led to the building of a 73-mile stone wall across the north of England, and to the definition of the Roman Empire.

The central question of these books is ‘how?’ How did a city state in central Italy manage to construct an empire that encircled the entirety of the Mediterranean, capable of flexing its muscles as far as Scotland in the north-west and the Farasan islands in the south-east? How did the Roman Republic, with its foundational suspicion of kings, come to accept the monarchy of the emperors? How did this form of government survive the clowns, crises and conflicts that beset it almost from the word go?

Holland’s approach to answering that question in all three of his volumes is to direct his reader to the top of the pyramid. These books concern themselves with the sifting and parsing of stories that purport to take the reader into the courts, into the bedchambers, and into the very minds of the emperors and their families.

The idea that several hundred years of history covering a couple of million square miles and who knows how many millions of subjects could be adequately told by the close study of a handful of powerful men seems, at first blush, somewhat old hat. It is, after all, hardly a recent trend to look for the marginal, the subaltern and the granular when seeking evidence for how a society actually functioned in a given historical period, how life was actually experienced by its denizens. Against the backdrop of all the archaeological discoveries of the last 250 years, what can one learn about the Roman Empire by retelling the stories that Gibbon had learnt by heart before he even began his Decline and Fall?

Holland’s goal in these books is only partly to explain; his desire to entertain can be compared to that of the emperors putting on a show in the arena. He positively leaps at the opportunity to use his vivid and engrossing prose to bring to life Suetonius’ salacious tales of imperial sex and bloodlust, to plunge the reader anew into the paranoia and cynicism of Tacitus’ Annals. However familiar you might be with Holland’s source material, you cannot but be arrested by his ability to conjure the terror of Vesuvius’ victims, the awe of a Roman legion’s first transgression of the English Channel, the unleashed savagery of Sejanus’ fall.

Knitting together this vivid collection of landscapes and portraits, however, is a crucial insight into the operations of the Roman Empire, one that justifies the author’s close and constant focus on the life and characters of the imperial court. In Holland’s eyes, the emperors of Rome were many things, not least narcissistic, bullying, bed-hopping, mother-killing, infanticidal demi-gods; but what really unites this cavalcade of emperors was the fact that they were, by and large, powerless.

That statement obviously requires some qualification. Hadrian’s globe-limiting buildings were not the creations of an impotent figure, any more than were Augustus’ annexation of Egypt or Julius Caesar’s invasion of his fatherland. What is abundantly clear from Holland’s narrative, however, is that such bursts of achievement are the exception. What, after all, do we see as we are taken on our gruesome tour of the imperial engine room? The Palatine of these books is certainly a hive of activity, but the manic levels of industry described here are almost all channelled in one direction: the manufacture and maintenance of the emperor’s public image.

The actual administration of the empire takes place largely off-stage. Towards the end of Pax, we are presented with a representative vignette of how the denizens of a middling city in a distant province experienced the power of the Roman state on a day-to-day basis. To say that it was experienced at arm’s length would be an understatement. Almost wholly absent from the lives of the Bithynians is the might of the legions, the rods and axes of the imperial governor. The men running the show in a Greek polis such as Prusa or Nicomedia are not imperial apparatchiks dispatched from Italy, but local figures with distinctly un-Roman names: Aristides, Philopappos, Dio Chrysostom.

Whether a tyrannical micromanager, like Domitian, or a laidback playboy, like Otho, wore the purple in Rome, it was left to these local aristocrats to intrigue and arm-twist, to build alliances and aqueducts, to experience acclaim and exile. Only an absolute crisis could stir the emperor to make a direct intervention in the affairs of his empire. And while that might take the form of a brutal military crackdown (as we see in the merciless savagery of the Judaean revolt’s repression), it is clear from the narrative that this was a weapon of last resort in the managerial arsenal. Far more representative of the imperial administrator was Pliny the Younger, the man dispatched by Trajan to Bithynia to investigate the financial crisis swamping the cities run by Eumolpus and Dio. Holland’s Pliny is quite rightly no crusading general at the head of a legion. He is rather presented as something akin to a senior partner from McKinsey, a management consultant politely but firmly asking to read a struggling company’s balance books, trying to see how it got itself into such a mess, and how it might dig its way out. His job done, Pliny or another of the emperor’s men will not stick around to take personal charge of affairs – the job of running the show returns to the local elite.

It is a rare thing to find an emperor in these books driving any particular policy in the administration of his empire. Tiberius has the wisdom to halt his predecessor’s financial rapacity (‘I want my sheep shorn, not flayed!’), Domitian understands the role of the imperial purse in curbing inflation, and Hadrian knows that it’s rarely worthwhile to prosecute a war in Iraq. By and large, however, imperial interventions are more likely to bring chaos than order, and the majority of these emperors leave things well alone.

For that reason, it might seem perverse for Holland not to have crammed his books on the Roman Empire with every Dio Chrysostom and Aelius Aristides he could get his hands on, to stage Hamlet without the prince. Paradoxically, however, explaining why this decentralised model actually worked for the Roman Empire requires the reader to understand the seemingly powerless centre.

The key insight of Rubicon holds true in Dynasty and in Pax: whatever the Greek philosophers might have hoped, the Roman elite did not seek power as a means to achieve lofty ideological goals; power was an addictive substance upon which Rome’s aristocrats were hooked, one which they pursued for its own sake, a craving that no dose could satisfy. The aristocrat’s day began with a morning hit – as he arose from his bed and stumbled bleary-eyed down the stairs, he was greeted by a hall thronged with clients seeking favours for their petitions. Granting the wishes of his dependents on a daily basis kept the tremors at bay until a grander opportunity to satisfy his hunger was granted – the ability to command troops, to slaughter and to enslave, to march through the streets of Rome in triumph, rivalling the prestige of Jupiter himself.

Such an approach to government found growth simple, but necessarily struggled with achieving stability. Prestige was a zero-sum game – one general winning a glorious command guaranteed the disappointment of another. Eventually this was bound to lead to a civil war cataclysmic enough to demand a fresh approach.

The sequel, Dynasty, showed how Augustus, Rome’s first emperor, succeeded in controlling and directing the awesome and destructive force of the aristocracy’s addictive appetite to excel. The genius of Augustus, presented not unjustly as the most uniquely able politician the world has ever seen, lay in his ability to transform himself from an addict enslaved to a hunger for power, to a rational dealer capable of satisfying the cravings of a subordinate elite entirely dependent upon his dispensary.

Holland emphasises the fact that Augustus’ pre-eminent position was entirely founded upon the sheen of his public image. The fact that he had won the civil war was a necessary factor for his pre-eminence, but it was not sufficient to make it last. Any number of warlords at Rome had temporarily held such positions. What separated Augustus from his predecessors and what allowed his supremacy to be measured in decades rather than years was a quality that the Romans had a word for but we do not: verecundia, the ability to judge what is appropriate and acceptable in a social situation. The metaphor Holland uses is that of the charioteer who knows precisely how much speed he can carry into a corner and precisely how hard he needs to pull on the reins to prevent his car from pitching and rolling disastrously off the racetrack. Augustus knew in his gut just how much royal imagery the Roman people would accept, just how much subordination the elite could bear, and just how many grams of power would satisfy their cravings without bringing on an overdose. Minuscule course corrections kept the show on the road for so long that with his death, few were left who could remember that politics had ever been transacted in any other way.

Pax picks up where Dynasty left off, seeking to explain how the Roman Empire’s novel monarchical system was sustained in the absence of the man in whose image it was fashioned. Taking up the tale with death of Nero and the tumultuous Year of the Four Emperors proves a sensible starting point. On Holland’s reading, Augustus’ immediate successors (Tiberius, Caligula, Claudius and Nero) had been carried to the throne by the momentum of Augustus’ reputation: a place in the imperial household and, ideally, the divine blood of Augustus in your veins was enough to justify a Julio-Claudian’s grip on power.

With the death of Nero, however, and the extinction of the Julio-Claudian line, Rome was confronted with a society that could not function without a monarch, but had no settled mechanism or criteria for who should fill that role. The arch could not stand without its cornerstone, but how to choose just one replacement from the abundance of rocks lying around? The story of Pax is essentially a series of trial runs: various candidates for the top job either push their way to the front of the queue or (more often) find themselves pushed there, and then do their best to impersonate an emperor, with deadly consequences should they fail.

Here the metaphor of the charioteer reappears: the emperor clings on for dear life as he attempts to balance the competing and often contradictory expectations of the key stakeholders. Some put their political capital into entertaining the restive urban plebs, others richly reward the Praetorian guard; some stake their reputation on the conquest of foreign lands, others put on miraculous displays to elevate themselves to the level of the gods. Either these apprentice emperors whip their horses into a frenzy and end up steering too far to the left or right, oversteering or understeering their chariot into the walls of the hippodrome; or they try to tread cautiously around the circuit and are overtaken and supplanted by ambitious rivals.

At most, Holland identifies three emperors who successfully managed to play the part of Augustus for any length of time. Vespasian, Trajan and Hadrian are the exceptions; the fates of Caligula, Vitellius and Domitian are the rule – men forced into increasingly caricatured roles as they desperately attempt to justify their rule by playing again and again their strongest suit (Caligula the conjuring thaumaturge, Vitellius the generous benefactor, Domitian the unforgiving moralist). Eventually the crowd grow weary of the performance, the player is booed off the stage, and the next actor is dragged on in his place.

Although the theatrical analogy is the one most consistently employed in these books, the role played by the emperor in the administration of his empire most closely resembles that of a CEO. The day-to-day running of the various arms of the company are far too many and far too technical to be personally overseen by the person at the top. On a given day, the vast, vast majority of the company’s activities will be carried out (or will fail to be carried out) whoever the CEO is or whatever they happen to doing that day. Just so, the local elite of an Alpine town will keep the city’s sewers in good nick regardless of whether Trajan or Nerva sits on the throne. The emperor exists to provide something far more important than technical proficiency – he is there to give the shareholders confidence in the system.

Since its earliest days, Roman government had developed as a hierarchically ordered network of patrons and clients. Should a problem arise, the affected party would petition their personal patron for help in fixing it. Should the patron be unable to resolve the issue himself, he would beg the favour of his own patron to help him. Up the chain the petition went until it was delivered to somebody with the power and resources to resolve it. The problem being solved, goodwill and gratitude would flow back down the system, boosting the prestige and status of everybody who helped the petition on its way; and putting every recipient of favour in the debt of the person who helped them out. Not for nothing are the chapter titles of Dynasty taken from the world of The Godfather.

The emperor’s role in this system was crucial – he was its guarantor. He was the ultimate recourse, the gold standard, the assurance that whatever the magnitude of the problem might be, there will always be somebody in the system with the resources, wisdom and power necessary to resolve it. The emperor’s centrality to this system of petition and response is best illustrated by a story told about the emperor Hadrian by the historian Cassius Dio. As Hadrian is out with his retinue inspecting his empire, an old woman pushes her way to the front of the crowd that has gathered to watch the imperial procession go by. As she tries to hand a petition to the emperor, Hadrian pushes her away, declaring that he does not have time for her problems. “Then do not rule!”, she shouts at him. Hadrian is stopped in his tracks. He turns back, takes her petition, and hands it to one of his secretaries.

Pax is frequently translated as ‘peace’, but the Latin term is closer to ‘pact’, and that word is fundamental to what the Roman Empire offered its citizens. The empire was an agreement, a covenant. It was a guarantee that whoever you were and wherever you lived in Hadrian’s great garden, you could entrust your fortune and your family’s safety to the care of a pedantic little man like Dio Chyrsostom. If you cultivate his good will, help him out when he asks you a favour, he will give you your daily bread and sort out your daily problems. And in the event of a cataclysm, the emperor is pledged to pick up the phone to him.

This is why the stories of the personal lives of the emperors have survived, and why the history of the Roman Empire cannot be told without them. Who could have faith in a system of government where the buck stopped with a buffoon unable to control his nymphomaniac bride? The circulation of stories about bad emperors and their subsequent downfall offered a peculiar kind of reassurance to the empire’s citizens – there is nothing on earth that will preserve an emperor who is not up to the job. Pax, as Tacitus said of this period, verum cruenta. Tom Holland has shown us that the Romans had peace, but a bloody one.