MANY OF Woolf’s questions are very old ones, which does not mean they are easy to answer. No one has yet come up with a satisfactory explanation of why Rome expanded as it did. This question isn’t as widely addressed in popular works as the cause of Rome’s fall, but in both instances, it is easier to explain how it happened than to explain why. Thankfully, no one suggests that the pre-Roman world simply transformed itself into the Roman Empire, but a good deal of work has been done to show that some regions were already changing in ways that made it easier for them to plug into the Roman system. In Gaul, for instance, some tribes were developing into something not too dissimilar from the city-states of the Mediterranean world, and many were producing a substantial agricultural surplus, encouraging trade while incidentally providing sufficient food for the conquering legions when they did arrive. Yet the Romans also conquered areas where this was not true, and it is too facile to claim that the empire stopped expanding when it reached communities too undeveloped to absorb.

Trade usually long predated military contact, as Rome’s flourishing economy influenced markets far beyond the provinces it physically controlled. Roman merchants were active in most areas long before the legions arrived. Caesar found them in the towns of Gaul, but more often, we only hear of their presence when they were massacred by the locals. Under the republic, Roman senators were forbidden by law from investing in large-scale trade or the companies fulfilling government contracts—the publicani or publicans of the King James Bible. They got around this in various ways, mainly through using freed slaves as agents. Former owners had considerable legal and social control over their freedmen and freedwomen, and many aristocrats were involved in numerous projects to spread the risks they took—the closest the Romans came to the idea of limited companies.

Yet, unlike more recent empires, trade did not in itself drive Roman conquest. Glory and plunder were at least as important, feeding political competition, and the leaders in Rome’s wars became fabulously wealthy and increasingly powerful as the empire emerged. In the longer run, this intensified the competitive nature of the republic’s public life and helped to make it violent. Men like Pompey and Caesar conquered great swathes of territory but also doomed the republican system with their civil wars. Emperors feared internal rivals far more than foreign enemies and were reluctant to let senators gain too much glory or win the loyalty of the legions. With a few exceptions, the rule of the emperors brought expansion to a standstill, at least after a final surge under Augustus.

The frontiers were static, but merchants still traveled far beyond them. They brought back amber from the Baltic and exploited the monsoon winds to sail to India and back. Some may even have reached China during the second century AD; the two great empires in the world were dimly aware of the other’s existence, even if most contact was through intermediaries. Roman goods often turn up far beyond the frontiers. Indeed, more Roman swords have been found outside the empire than within its borders, the bulk of them in Scandinavia. Some of this resulted from simple trade and some from open warfare. A few finds were clearly seized in raids on the empire. Other contact was a mixture of commerce and diplomacy. Spectacular finds of silver, gold and glass ornaments suggest gifts to tribal leaders, quite possibly sponsored by the state to keep them peaceful. Rome’s influence stretched far beyond its frontiers.

The empire’s great market had, at times, a drastic impact on peoples outside of Rome’s confines, and intertribal warfare may well have increased simply because of the proximity to the empire. Raiding intensified to provide slaves for sale within the empire in return for luxury goods—just as some African communities turned to warfare to supply the demands of Arab and European slavers in later centuries. Gifts of money and weapons to friendly leaders led to some carving out great kingdoms for themselves. A few became so powerful that they were seen as threats to the empire. Changes in Roman subsidies—or equally the movement away of frontier garrisons that had formerly provided a market—could create hardship and desperation in external communities and might well prompt them to raid the empire instead. Frontier relations were a delicate balancing act.

ROMAN WARFARE was always accompanied by diplomacy, and the formal submission of an enemy was as glorious a success as beating him by force. Yet ultimately military force made the empire possible. The Romans often went to war, but then so did almost every people and state in the ancient world. Greek city-states almost seem to have considered hostility a natural condition of interstate relations. The Romans liked to see all of their wars as just, defending themselves or their allies from real or threatened attack. Sometimes the allies were acquired only a very short time before the war commenced by militant Roman leaders eager for glory.

Julius Caesar took great pains in his Commentaries on the Gallic Wars to show that all of his campaigns in Gaul—and across the Rhine into Germany and over the sea to Britain—were for the good of the republic, and by Roman standards they probably were. He casually talked of pacifying—the Latin verb is pacare—the tribes of northwestern Gaul who had little prior contact with Rome but who had not treated his envoys with suitable respect. The Romans did not grant other peoples any rights even vaguely equal to their own, although Caesar would remark that it was natural for all men to fight for freedom. Rome’s freedom and advantage simply trumped the interests of others.

Yet none of this thinking was probably unique. We are simply better informed about Roman attitudes than those of other ancient peoples. As Woolf points out, the Romans considered pietas—a far stronger word than our piety—to be a characteristically Roman virtue. The Romans took care to worship the gods correctly and reliably, on the whole respecting and even adopting foreign deities. There was a formal rite performed outside a besieged city to invite the gods of that community to leave and come to new homes prepared by the Romans. Rome’s success was seen as coming from this divine approval, but Woolf shows that this does not explain Roman expansion, and there was certainly no sense of a crusade. Virgil’s Jupiter announced that it was the Romans’ destiny to “spare the conquered and overcome the proud in war”—parcere subiectis et debellare superbos. This neatly divided the world into those who had already submitted and acknowledged Roman might, thus deserving a degree of mercy, and those yet to be defeated. Yet this, like the promised dream of imperium sine fine, or “power without limit,” did not produce constant or consistent expansion.

The Romans certainly took warfare seriously, almost personally, and in this, they were unusual. The Roman Republic devoted itself to war in a way unmatched by any of the rival great powers, whether the mercantile empire of Carthage or the kingdoms of the Greek world. The fleet that won the final battle in the twenty-three-year First Punic War was paid for by voluntary contributions made by individual Roman aristocrats. The Romans accepted staggeringly high casualties—a third of their three hundred senators between 218 and 216 BC died at the hands of Hannibal, and fifty thousand soldiers were killed and twenty thousand captured in a single day at the Battle of Cannae. Yet Rome would not concede defeat in the face of these grim totals. The Romans did not give in, kept fighting and learned from their mistakes. In the end, it was the enemy who gave up and sought peace. Rome had the resources of manpower to absorb such appalling losses, and that was rare. But the determination to persist in a conflict until either the Roman state was destroyed or the enemy threat was permanently removed—by becoming a clearly subordinate ally or ceasing to exist as a political entity—was unique. The Romans expected victory to be permanent. A piece of graffiti scratched onto a cave wall in Jordan reads: “The Romans always win. I, Lauricius write this, Zeno.” The context is as unclear as the identities of the two men, but it was not a bad summary of Roman warfare.

The Romans were good at winning wars and even better at creating lasting peace on their own terms—the losers permanently pacified. Rome had the manpower to survive Hannibal’s onslaught because by this time, it had absorbed almost all of Italy and its various cities and peoples. Some became citizens, and all were allies who willingly fought for Rome and had a share in the rewards of victory. It was rare at any period for a Roman army to consist of more than 50 percent Roman citizens, and often the percentage was much lower. The bulk of the population throughout the empire was descendants of the peoples conquered by Rome, many of whom became citizens and adopted Roman lifestyles. The senator and historian Tacitus wrote of British aristocrats wearing togas, learning Latin, and building basilicas and villas, calling such things “‘civilization,’ when in fact they were only a feature of their enslavement.” The cynicism veils the truth that the Romans excelled at making conquered people into Romans.