Notice the title of Peter Heather's fascinating study of the final centuries of the Roman Empire. It is a clear tribute to Gibbons, yet the "Decline" is intentionally missing. Because according to Dr. Heather the Roman Empire never declined; its fall was due to external, rather then internal, forces, and the perpetrators were two: the Huns and the Goths.
Heather rejects the theories that see the cause of the fall of the Roman Empire in internal maladies. Contra popular opinion, he argues that the division of the Empire to Western and Eastern parts was rational given the increased size of the Roman population. As the Roman way of life spread, and more and more conquered people became Roman citizens, the patronage that had to be distributed became too enormous for any single Imperial Court - hence, the need for two Courts.
Nor is the fault in the Christianization of the Empire; although he acknowledges that the rise of Christianity brought a Cultural Revolution (separation of the Living from the Dead; Equality of all before the Lord; diminished importance for the educated Romans in comparison with the simple true-believers, pp. 121-122), Heather doubts it effected the functioning of the empire much. The Roman Empire was still perceived as divinely blessed "only the nomenclature was different" (p. 123), Christian theology fitted neatly into Roman Chauvinism, and it was only as consequences of defeat that St. Augustine started to develop his anti-Nationalist theology (pp. 230-232).
The best evidence against the "internal decline" thesis is that the Roman Empire did not actually collapse - only it's western half did. In the East, the Roman Empire soldiered on, until another powerful foreign threat - Islam.
Therefore, Heather suggests, the answer is external: As a consequence of the exposure to the Roman world, the Germanic tribes confronted by the Romans have changed. An agricultural revolution took over the German world, increasing its population and changing its organization: along with surplus, there developed inequality, with powerful leaders and kings solidifying larger and larger groups of so-called "Barbarians" (pp. 87-94).
But the grows of the German population was not in itself, enough to shake and eventually to topple the Western Empire; the fuse for that was a new menace, coming from the East - the Huns.
Heather remains officially agnostic as to the origin of the horse riding people from the Great Eurasian Steppe, although he seems to support the theory that their origin related to the Hsiung-Nu - a Nomadic threat to the Chinese Empire several centuries before (pp. 147-149).
The Huns made their ways into the neighborhood of the Roman Empire in two stages - in the late 4th century, they have arrived at the Caucasus, and in the second quarter of the 5th century to East and Central Europe, culminating in the raids on the Western Empire, by their sole unifier and greatest leader, Attila.
But it was not ultimately the Huns who destroyed the Empire. What the Huns did was trigger a chain reaction of migrating "Barbarians" the greater, richer and more unified Germanic people who fled into the Roman Empire. "As Germanic groups moved on to Roman territory to escape Hunnic aggression, this long standing process acquired new momentum. One of the most important ... phenomena of the fifth century narrative is that all of the major successor states to the west Roman Empire were created around the military power of new barbarian supergroups, generated on the march"(p. 451).
As the Roman Empire faced these threats, it suffered from a vicious circle of damages; the more the Goths invaded the worse the empire's capacity to raise taxes became, thus turning the Empire weaker and more tempting target. The Loss of Africa to the Vandals was a particularly hard stroke in that regard. And every time the empire seemed to be able to overcome one crises, the continued advanced of the Huns pressed new waves of invaders into its boarders, undoing the Roman effort. "[T]he various crises faced by the western Empire ... represented no more than the slow working-out of the political consequence of the earlier invasions" (p. 434)
This short synopsis does not come close to doing justice to Heather's sophisticated and fascinating account. Yet in blaming the fall on an "Exogenous Shock" (p. 450), I think Heather may be ignoring one major change in the Roman Empire - its relative lack of belligerency.
As Heather tells it "Roman expansion was driven by the internal power struggles of republican oligarchs... and by the early Emperor's desire for Glory." But eventually, the provinces that the empire started to conquer were just too poor to be worth conquering "The Roman advance ground to a halt... around a major fault line of European socio-economic organization"... it was not the military prowess of the Germani that kept them outside the Empire, but their poverty" (pp. 56-58).
But as the agriculture revolution took over the Germanic world, did not that arithmetic change? If the Roman Empire's border was initially determined on economic cost/benefit grounds, it seems to have been perpetuated by tradition. New threats lurked in the dark forests of Germania, but new opportunities were there, as well. Why didn't the late Empire move to take advantage of the opportunities? To me, it seems that an answer to that is essential for the discovery of the causes for the Fall of the Roman Empire.