The Roman empire grew by conquering and absorbing neighboring peoples, one after the other. The subjects in the provinces lost their identity as peoples and became, if they were lucky, Roman citizens. All this was ignored by the older scholars who identified with empire builders and admired Rome. Modern Europeans, on the other hand, who disapprove of foreign conquest and the suppression of liberty, still find much to praise in the principiate because it is assumed to be a period of defence of the provinces against barbarian invaders, since modern states may legitimately defend their borders against attack. . . .

It is often ignored by Roman historians that the forces of an empire never have the sole duty of defending the border. They are armies of conqest and occupation, as well as defence. The work of an army of occupation is basically different from that of a fighting army, but there is not usually a single, clear-cut process of reorganization which marks the transition. The work of an occupying army is rarely morally edifying in terms of military glory or plain human decency. It requires very special checks and balances for an occupation to be civilized in modern terms. These did not exist in the Roman empire. Roman historians should not overlook the fact that the Roman army was in many areas and periods an army of occupation or an internal police force and should try to realize the consequences of this for those who suffered occupation. A further essential function of the Roman army was the maintenance in power of the ruler. The many civil wars that were fought cannot simply be treated as so many irregularities. They were the result of a situation in which only the army kept an emperor in power and only a rival with an army could bring him down.

A view often encountered in histories of the Roman empire is that Rome, in the first and second centuries, created static lines of defence and thus passed to a rigid strategic defensive. As a result the 'barbarians' slowly gained the initiative. . . . [But the] political boundary of the empire was irrelevant as a concept, and the military boundary was never organized as a 'line of defence.' The limits of the empire, if at all defined, were expressed in terms of power and military action. The only clearly demarcated boundaries were those of the provinces. . . .

A related subject is the offensive or defensive character of Roman diplomacy and warfare. It is often very hard to determine whether an army is engaged in defence or offence. . . . Historians of the principiate often assume that every move by the Romans was aimed at establishing a secure frontier. Even if it were true . . . that is a view which oversimplifies. Two parties are needed to fight a war, and both have motives. While a war is being fought the aims of both sides will change in accordance with developments in the field, and whatever is achieved may be completely different from what was anticipated.