From my reading list:
Roman government seems to be the subject of many misconceptions. It was in constant flux over a very long period of time, and did not attain any great measure of homogeneity, except, perhaps, briefly in the age of the Antonines. Its undoubted success was due to the limited but clearly defined goals that were set. It provided magistrates to settle disputes and to exact tribute. It provided an army for external defence, law enforcement, and internal security. And it supported the authority of approved local or regional elites, often through their participation in religious rites and civic ceremonies. The magic combination involved both great circumspection, in the degree of the state’s encroachment on established rights and privileges, and utter mercilessness, in defending lawful authority. (Davies, Europe, pg. 171)
By the mid-third century the Roman Empire was showing all the outward symptoms of an inner wasting disease. Political decadence was apparent in the lack of resolve at the centre, and in disorder on the periphery. In the ninety years from ad 180 no fewer than eighty short-lived emperors claimed the purple, by right or usurpation … The army dictated to its civilian masters with impunity. The barbarians flooded over the limes [frontier line], often unchecked. The raids of the Goths turned into permanent occupations. (Davies, Europe, pgs. 191-2)