Quote Originally Posted by athanaric View Post
There are many reasons for that. AFAIK a population increase in Germania and other regions bordering the Roman Empire that put more pressure on the borders, internal social issues and strife (both political and religious), increasing decadence in Rome proper, mass migrations that were part of a larger population shift encompassing most of Europe and Northern Asia, and so on. The Romans tried to handle these problems and they succeeded in Romanizing quite a few Germanic tribes, but in the end it was too much.
There is no real evidence for the assumed "population pressure" in Germany across the Rhine. "Decadence" doesn't explain anything; a society's morals have little to do with whether a given state is militarily powerful or not. And the so-called "Migration Period" is a misnomer; the fifth century did not feature a substantially greater amount of movement of peoples than did certain previous centuries, and probably involved less migration in Europe than did the first century AD. While migratory activity happened, it was less relevant to the demise of the Roman Empire in the West than other factors, chiefly the Romans' series of systemic civil wars between the imperial government and Gallic interests that effectively destroyed the Roman military as an effective fighting force and promoted the rise of alternative areas of legitimacy to the Ravenna government.

Arjos had it basically correct. The Roman army was the real genesis of these so-called barbarian tribes, e.g. the Franks, Goths, Sciri that "settled" on Roman soil and established independent states. Not because the Roman army was comprised of "barbarians" who decided to put the woad back on and start speaking Celtic (or Gothic, or whatever) as soon as the Roman Emperors started getting weaker - the number of "barbarians" in the later Roman army has always been hotly contested, is nearly impossible to know, and is furthermore a largely irrelevant question. Ethnicity was not determined based on ancestry. The Franks of Clovis weren't Franks because they were descended from tribes that had lived outside the Empire, they were Franks because they were soldiers in the Roman army on the Loire River and chose to identify as a Frankish constructed ethnicity when the Roman Emperors ceased to be relevant in Gaul as simply the last and most obvious step in the progressive segregation of the Roman army and its identity from Roman civil society. The Goths of Alareiks weren't Goths because they had crossed into the Balkans with the Tervingi and Greuthungi; they can't have been, because it would be numerically impossible. They were Goths because they served in Alareiks' army units and supported his bid for increased authority in the Roman army in general and in Honorius' government in particular. And so on, and so forth.