The Roman army seems curiously modern with its professionalism, structured bureaucracy and detailed military organization. Indeed, John Keegan has called it the ‘mother-house’ of modern armies. It is not surprising that this army, which had such an impressive record of success, has often played a part in modern analysis of the nature of war and the impact of military practices and warfare on society. However, the Roman army does not fit easily into any pattern or theory derived from analysis of the armed services of communities in different eras, and its diversity and close links with a unique society and culture make generalizations very difficult. Furthermore, we should remember that the army of the imperial period had been initially moulded to suit the wishes and needs of one man, Augustus. He determined its size, structure, disposition and command, and in this he had at least one eye on the straightforward matter of his own survival. Augustus engaged in frequent warfare and kept personal control of his army, which he stationed permanently in the provinces, especially Germany, the Danube lands and the east. In this he influenced the direction of Rome’s military and strategic interests for generations to come, and indirectly the future course of European history. Yet he managed to combine his proclaimed role as a great conqueror with a system of government largely free from the trappings of military autocracy.
From this complex legacy emerged significant features that were to have enduring importance: above all, the idea of professional, specialist soldiers paid by the state, who earned rewards by their service and an entitlement to a kind of pension on discharge. Increasingly legionaries were recruited from outside Italy, and large numbers of non-Romans were also accepted to play an integral part in the military as auxiliaries. On completion of their service these men received citizenship for themselves and their children, and there was a significant degree of assimilation and integration into the Roman way of life. There was also an avenue of social mobility through promotion in the army to the rank of centurion and above. It was a conspicuous achievement to recruit this army largely from subject peoples and to preserve its loyalty and commitment for over four centuries in the west.