[Rockingham]

Quite true. The Roman Empire had an excellent road network, a superb and high adaptable military organization in its legionary system, and (usually) an effective central administration. Moreover, the Roman way of life proved highly attractive to the barbarians that it incorporated or who settled on its frontiers. A century or two of peaceful contact and trade often sufficed for Roman goods and way of life to turn barbarians into friends or even allies who sent their sons into the recruitment system of the Roman Army. You may find the Edward Luttwak's The Grand Strategy of the Roman Empire and The Grand Strategy of the Byzantine Empire of interest.

[SunkenCiv]

Having no need to protect themselves for hundreds of years made the still rather large population of the Roman Empire sitting ducks when, for example, the legions were withdrawn from Britain. The pickings in the Empire were richer than those outside, and the mass migrations of quite a number of different people led to a century or more here and there of various successor states of different sizes and cohesion. The Vandals wound up in modern Tunisia after ripping through during the 5th century. And of course, the Empire had already gone through nearly all the 3rd century broken into schismatic mini-empires, partly in response to, and partly as a cause of, barbarian migrations. During the 3rd century there was a blossoming of free market commerce and local autonomy that more than set the stage for the "Dark Ages" / medieval division into small city-states and principalities. In the 6th c Procopius lamented Justinian's campaign to reconquer the western provinces. The feudal society developed as a consequence of the need for common defense and also the need for the self-appointed nobility to boss everyone around. :^)