The old explanation (going back to Mommsen) centers on the idea of "defensive imperialism" -- that is, that Rome was in essence not aggressive by nature, but was strongly motivated to secure her own interests and respond in kind to external threats. It was a mandate which was thought to have its origins in the very beginnings of Rome, born in the chaotic interplay of self-interested forces in central Italy. As Rome defeated neighbors, she came in contact with further actors beyond those borders, and then even more distant actors. As Rome's power grew, so the theory went, the magnitude of threats to her interests also grew; at last it was necessary to assume direct control of territory outside Italy for logistical reasons. If you read Caesar's Gallic Wars, you can see how such a theory would come to be: at every step of the way, he is careful to explain why each new action, pushing further and further away from his proconsular mandate, is a necessary defense of or punishment for the transgression of the dignitas of the Roman people. His initial fight, with the migrating Helvetii, was in response to them crossing the Roman Province and attacking a Friend and Ally of the Roman People. Should Rome not respond in kind, it would be sending a message to all the Friends and Allies of the Roman People, right? After that, every little Gallic tribe that got involved was also announcing their intention to go against the interests of the SPQR, and they too had to be corrected. Oh, the Britons came over to the continent? They must also learn.

This theory has been dropped by all but the most stubborn scholars these days. The notion of the Romans as fundamentally peace-loving folk, being forced into a constant string of defensive wars, does not hold up under a lot of scrutiny. Most recent studies have embraced the notion that the Romans were fundamentally belligerent as a society: one of the basic measures of aristocratic prestige was positive performance on the battlefield. This is not unique to the Romans, of course, and in fact nearly all Indo-European societies had at their beginnings a valuation of military prowess in some way. The Roman system, however, was a sort of feedback loop for military glory. Roman aristocrats built their prestige on it as young military tribunes, rose through the cursus honorum by riding in part on those accomplishments, and when they attained the consulship and inevitably brought legions against a foreign power of some sort (successfully or not), they then entered the Senate and, with all the other old men who had led legions, used their influence to perpetuate the system. Military accomplishment was absolutely essential to the Roman aristocratic ethos, and armed prowess permeated every aspect of Roman society. Harris (*War and Imperialism in Republican Rome, 1979) has a (now) dated but excellent conception of the Romans as aggressors.

For no other ancient society (excepting perhaps Sparta) was armed conflict and the State so intertwined. Let me quote Serrati when he writes that "warfare bonded all ranks of Roman society together and was an integral part of the state’s social, political and cultural life. Warfare not only benefited the Roman state, but it also crossed monetary divides and linked together Romans of different classes. It therefore follows that the Roman state was imperialist, and that the Romans themselves favoured war as an externalization of their socio-political system." Now "imperialism" is a 19th century concept, and its application to the ancient world has evolved right along with the modern understanding of the concept. For the colonial powers, the focus was on how Empire was acquired and under what justifications (see 'defensive' imperialism, above). Harris (and others at the time) brought 'post-colonial' perspective and reminded the world that the Romans liked war, like the benefits and treasure it brought, and were happy to call it 'just war' by whatever convenient means were at hand. Between the outbreak of the Second ("Great") Samnite War in 327 BCE and the end of the First Punic War in 241 BCE, the Romans led legions (and dragged along attached allied contingents) in all but five years. That's 81 years of combat and 5 years to take a breath. There were 74 triumphs celebrated in Rome during this period.

Not all of Rome's conflicts were aggressive, of course. They were fighting for their right to exist against the Gauls in the early 4th century BCE, and Carthage was certainly a recalcitrant and dangerous imperialist opponent on their southern border at the dawn of that conflict. The general trend, however, is clear: warfare was intrinsic to how the Roman state was organized -- even the citizen assemblies were arranged in military terms, in their beginnings! Their politicians were field marshals, and their aristocratic ethos centered around being a successful politician. As a society, from the top to the very bottom Rome reaped the benefits of conquest and were unabashed in that enjoyment. The basic system of Roman relations with defeated foes, especially in Italy, did not seek to render them impotent, but instead maintain their fighting ability to be mustered as allies for still further conflict. Every time an Italian enclave was defeated and added to this system, the Romans got stronger and the rest of the world got a little weaker. In basic terms, it was a system which constantly pushed the borders outwards in all directions. By the end of the reign of Augustus, Rome had seen her borders pushed to the Rhine and Danube in the north, the Atlantic in the west, the Sahara and down the Nile in the south, and the Euphrates in the east. For a 'defensive imperialist,' this suggests that for the first time, there were no Empire-level threats left (ignoring the Parthians), and so the defensive empire strategy began to wind down and we get the pax Romana. Others argue that a full century of civil war had simply exhausted the Romans, broken the self-feeding system, and realigned Roman priorities. But that's another topic!