The genius, if we may call it that, of the Roman Empire was based on the ability of the Romans to bring a wide diversity of peoples into one political unity, founded in military conquest and political and economic control, but eventually welded together into a psychological unity where people throughout the empire looked upon themselves as citizens of Rome, sharing in the governance, and financial success of the Roman Empire. To be sure, there were outlying provinces where the separate identity of the various peoples was retained, but in the main, the Empire was able to overcome this sense of separateness and create the Roman identity. Those parts of the Empire which were not so perfectly assimilated, such as the people of Britain or the Jewish people of the Palestinian region, nevertheless were kept under control and at least partially assimilated.
The failure, however, is the flip-side of this success. Sri Aurobindo notes: “By crushing out, however peacefully, the living cultures or the incipient individuality of the peoples he ruled, he deprived these peoples of their source of vitality, the roots of their force. No doubt he removed all positive causes of disruption and secured a passive force of opposition to all disruptive change; but his empire lived only at the centre and when that centre tended to become exhausted, there was no positive and abounding life throughout the body from which it could be replenished. In the end Rome could not even depend on a supply of vigorous individuals from the peoples whose life she had pressed out under the weight of a borrowed civilisation; she had to draw on the frontier barbarians. And when she fell to pieces, it was these barbarians and not the old peoples resurgent who became her heirs. For their barbarism was at least a living force and a principle of life, but the Graeco-Roman civilisation had become a principle of death. All the living forces were destroyed by whose contact it could have modified and renewed its own force. In the end it had itself to be destroyed in its form and its principle resown in the virgin field of the vital and vigorous culture of mediaeval Europe. What the Roman had not the wisdom to do by his organised empire,– for even the profoundest and surest political instinct is not wisdom,– had to be done by Nature herself in the loose but living unity of mediaeval Christendom.”
Sri Aurobindo, The Ideal of Human Unity, Part I, Chapter 6, Ancient and Modern Methods of Empire, pg. 46