The received wisdom concerning the fragmentation of the Western Roman Empire has undergone a total volte-face over the last century. From Völkerwanderungen, movements of entire peoples wearing furs and horned helmets violently invading Roman territory and carrying out ethnic cleansing, we are now expected to believe in "Elite transfers", small numbers of immigrants who came peacefully and amicably and took control with the full approval and cheerful willingness of the locals, and who apparently so impressed these indigenous populations that they all wanted to walk and talk like their new wonderful, kindly and not-at-all-oppressive rulers. This view is invariably accompanied by much sociological cant around concepts of "class", "status" and "identity" - the latter, so we are supposed to accept, being something which whole communities of mid-first-millennium peasants could and did suddenly change at the drop of a hat.
When there are two such opposed viewpoints, you can be sure that the truth is to be found midway between the two. Peter Heather, combining literary sources, archaeology and modern understanding of population movement and change, partially accepts some of the contemporary ideas, but persuasively argues for large scale migrations having often played a part in the changes across Europe in the period c. 400-1000. As Heather drily notes about the Slavs for example, but which equally applies to all the other invaders of the period, "Their military effectiveness makes it extremely improbable that [the changes] came about just because the indigenous populations thought it would be great to become a Slav."
It's a huge book, often repetitive, and certainly fairly hard going, but it's a well argued presentation and an extremely important contribution to the understanding of this period of European history.