As one of the decisive developments in western history, Roman expansion has invited continual reinterpretation by historians. Polybius, who wrote his history in order to explain to other Greeks the reasons for Roman success, believed that after their victory over Hannibal the Romans conceived the aim of dominating all before them and set out to achieve it in the Second Macedonian War. If one accepts the Roman view that they fought only "just wars"--that is, only when provoked--then Roman conquest emerges as "one of the most important accidents in European history," as Rome had to defend itself from threats on all sides. Historians have suggested other motives for empire, such as a desire to profit from war, an interest in commercial expansion, or a love of the Greeks, who asked for protection against Hellenistic monarchs. Major historical phenomena of this kind rarely receive final, decisive interpretations, but several assertions may be ventured. Some of the interpretations are anachronistic impositions on the ancient world; ancient testimony, for example, gives no support to commercial or mercantile explanations.

Cultural and economic interpretations seem more appropriate. Roman culture placed a high value on success in war: virtus (courage and qualities of leadership) was displayed, above all, in war, and the triumph, a parade through Rome celebrating a major victory over an enemy, was the honour most highly prized by the senatorial generals who guided Roman decisions about war and peace. Moreover, these leaders, and the whole Roman people, were fully aware of the increasing profits of victory; in the 2nd century commanders and soldiers, as well as the city itself, were enriched by the glittering booty from Africa and the Greek East. Yet, it is rightly pointed out, Roman intervention in the East was sporadic, not systematic, and the Romans did not annex territory in the Balkans, Anatolia, or North Africa for more than 50 years after their initial victories. The latter point, however, is not telling, since the Romans regarded defeated states allied to them as part of their imperium, whether or not they were under Roman provincial administration. The sporadic timing of the wars would seem to support the Romans' claim that they only reacted, justly, to provocations. But attention to the individual provocations should not blind the historian to the larger pattern of Roman behaviour. From 218 the Romans annually fielded major armies decade after decade. Rome was able to go to war every year in response to provocations only because it chose to define its interests and make alliances farther and farther afield. Polybius, as noted, reveals how the Romans were the masters of manipulation of circumstances to force opponents to behave in a way they could interpret as provocative. Therefore, the Roman interpretation of "just wars" and the Polybian interpretation of a universal aim to conquer need not be contradictory. The concept of "just war" may have justified any given war but does not explain the perpetual Roman readiness to go to war. For that the historian must look to Polybius' universal aim or to general political, social, economic, and cultural features of Rome. Finally, it must be remembered that in some instances it was clearly the Roman commander who provoked the war in order to plunder and to win a triumph (e.g., Licinius Lucullus, governor of Nearer Spain, in 151).