The following represents a quick shot by my friend, Paul Halsall to answer the question above in a way helpful to hist beginning students. It was made on MEDIEV-L list on Fri, 24 Apr 1998 18:29:04 -0400. I have slightly cut it.
 

Here is what I think happened in the middle ages (and I make no apologies for the term).

In the third century CE [common era] there was in the western Eurasian/north African land mass a large multi-ethnic state (or
Empire) in the lands around the Mediterranean. Despite the existence of several subaltern cultures [Celtic, Coptic, Syriac],
this area was united not only by a single state, but by a shared Greco-Roman culture among its elites. From the mid-third to
mid-fourth century Christianity became the common religion of roughly half the inhabitants of the area (this estimates an
expansion of c. 3.4% per year since c.50 CE). A major political and military crisis at the end of the third century was resolved
in such a way that the Empire could continue both as a state and as in cultural forms in most of its territory until the late sixth century.

The Big News of the sixth-seventh century is the splitting of the circum-Mediterranean empire and culture into three quite
distinct cultures.
 

• In the North-East Quadrant, The Eastern Roman Empire [Byzantium] continued the culture [Roman government/Greek Language/Christian Religion] of the late Roman Empire without any clear disruption until the mid-to-late seventh century. Until that time the civic network remained strong, and the class structure diversified. In the mid-seventh century, however, the loss of Egypt and Syria to the Muslims, and the increasing loss of control in Italy, meant the Byzantium lost much of its multi-ethnic character and became a largely Greek state. Moreover, repeated attacks devastated the civic structure, and most cities simply shrank to insignificance. Byzantium became a large agrarian state with one major urban center and a massively simplified class structure. This represented a change from the Late Antique world, and begins the journey of Orthodox Christendom as a distinct culture, one that continue until today in Greece, Russia, Serbia, etc.
• In the Southern half of the old Roman Empire, the very rapid conquest by Islam in the seventh century provided the basis for a distinct culture. In many respects the old culture provided major inputs - in science, architecture, coinage, government, philosop hy and religion. But the Arabic language, and a huge artistic and governmental contribution from Persian civilization must also be taken into account in assessing this new Islamic world. In a variety of forms, the Islamic culture which originated in the seventh century continues today.

• In the North-West Quadrant, roughly speaking, we what I will label "Latin Christendom", later known as "The West". In every measurable way [population, literacy, economic activity] this region was much more sparse than either Byzantium or Islam. [This is not to ignore tremendous regional variation.] From the early/mid fourth century on, political control in this region was wrested from Roman elites by elites which, however romanized, identified themselves by other affinities. In general a "Roman" culture does seem to have persisted in much of the area [although not all parts - see Britain for instance], in ever more ghost-like fashion. The strongest dose of Romanitas survived among the literate population, a population outside Italy that was increasingly likely to be made up of churchmen. In other words, and with caveats about the terms, we see a continued Latin/Roman element in the Church and a new "Germanic" element among the secular ruling elites. That certain features of Roman government, such as taxation practices, persist in some areas is not especially relevant [unless one thinks that the persistence of the Indian Civil service means that modern India is "British."] Based on assessable measures such as literary production, economic activity, archeological remains, one has every right to suggest that, in comparison to the continuing Roman culture of Byzantium, and the rapidly integrating culture of Islam, that "Latin Christendom" was by far the weakest of the three successor cultures. Whether this was due to a more or less slow erosion, or the catastrophic effects of plague is not especially significant. This breakdown of one Mediterranean culture into three successor cultures, all of which draw on the ancient roots, is, in my opinion, one of the great themes of world history. The differences and similarities between the successor cultures allows us a rare opportunity to suggest "tests" in which we can compare what happens in one culture/region with what happens in another. [For instance, why did the Eastern Roman state remain strong and centralized while the Western one foundered? How did the relationship of religious and political elites vary from culture to culture?]

What I want to explain is how the relatively sparse and unintegrated culture of the early middle ages became the ever
more complex and expansionary European culture we see from the late eleventh century on; and what were the salient aspects of this culture that laid the foundations for the world historical role of the "West":- that it was the first to establish a world
hegemony; and is still the culture which provides hegemonic political, economic and cultural ideas.  How did this come about?

Someone once proposed two alternative explanations that could be given: one, that Western European culture was in basic continuity with late Roman culture; the other than Western European culture represents a forced marriage between Latin culture maintained by the Church and a "Germanic" political culture. I am not really happy with either suggestion, mainly because, while evidence of continuity does not compel me, it is equally uncompelling to argue that it took 500 years [from Clovis to Otto I] for a modus vivendi between "Latin" and "German" elements to be reached.
 

• In political organization, there is no question that the memory and ideal of the Roman Empire had a continuing importance throughout the middle ages. The early Germanic kingdoms used some of the forms of late Roman imperial government; a series of attempted imperial revivals by German dynasties was predicated on the notion that the Empire was being restored; and the strongest medieval states until at least the mid-thirteenth century were these revived empires. What is crucial is that the effort to "restore" an empire ultimately foundered, and the two regions in which the imperial ideal was most frequently exercised, Germany and Italy, fell into a political and governmental mess in the thirteenth and fourteenth century from which they did not emerge until the late nineteenth centiry [if ever in Italy's case.] I strongly resist efforts to portray these echoes of empire as in "continuity" with the Roman empire, or even a suggestion that the Ottonian Empire was in "continuity" with the Carolingian.
• What is of significance for the future is the establishment of "national monarchies" during the 12th and 13th centuries in England, France, and Spain. These states are of special importance as much because of their future as because of the their power at the time. It was these Atlantic states after all, which were to succeed in establishing the most stablely-defined countries; earliest national consciousnesses; and which were to impact the rest of the world. [A world which is much more fluent in English, French, and "Iberian", than the languages of the foundering imperium, Italian and German.]
• Now the question arises, why did these national monarchies establish themselves and thrive, while the imperial ideal foundered. I agree there is no simple answer here. But, while not wanting to hinge the entire history of the world on whether there was peace or strife [better than anarchy?] in 10th century Aquitaine, it seems fair to argue that the weakening of centralized control in the late Carolingian era did so weaken royal power in France that a distinct type of militarized landed aristocracy became of special cultural and political importance.  One does not have to use the word "feudalism" in this context. A great deal of effort was put into controlling this militarized landed elite by the Church and by a reemergent royal power in both England and France. From this effort developed some of the salient institutions of the later middle ages - the knight, ideas of political consultation, and ultimately, in England, a legal system based on real property.
• Economics as much, if not more than, politics created the very vibrant Latin Christendom of the 11th century and later. Although I agree that there is some evidence of continuing economic activity at scattered sites, the overwhelming impression that by the late seventh- and eighth-century most of Western Europe was functioning at scarcely better than subsistence farming levels is hard to shake. Perhaps because the weather got better [!], population began to rise from the mid/late 9th century. Widespread adoption of technological improvements permitted surplus production, and we see, in areas where there were none, evidence of settled villages. More significantly we see an ever greater number of towns. Towns have many consequences for a culture [they permit multiple subcultures for instance], but perhaps the most dramatic was the monetization and commercialization of society. Even though urban populations remained small in comparison to rural figures, it seems to be the case that by the mid 13th century the European economy was a function of urban activity. Towns effected not only economics and culture but politics also: commerce and direct taxes provided central governments with new sorts of revenue, and permitted new types of politics. The representation of towns in the later English and Spanish parliaments is especially interesting in this regard.
• The repeated problems faced by attempted empires, contrasted with the persistence of ecclesiastical organization. The strength of the Church was in its fairly secure ideology married to its practical adaptability: to the basic pattern of episcopal organization a great variety of other forms of organized Christianity permeated European  life. A recurrent pattern was of ecclesiastical hierarchies which transcended political boundaries [think of the episcopal hierarchy, the Cluniac "order", the Cistercians, the various mendicants orders, and even the Templars.] I do not think this patter in seen as persistently in either Islam or Byzantium. What it meant was a quite real distinction [not "separation"] between church and state, and conflict between them. From this conflict, we can see the emergence of canon and civil law argumentation, and a method of education [research::organize::set principles of interpetation::reach conclusions] which was put to wider use in universities.

© Paul Halsall 1998