The overall picture seems to be a retreat by Germanic tribes into the west and the arrival of Slavic tribes, seemingly from a start-point in what is now eastern Poland, mixed in with further post-Hun invaders from various steppe tribes, from the Avars to the Magyars. Indeed, in a despairing variant, the elites of the original Croats and Serbs may have been speaking an Iranian language, which is the point where I think anybody sensible just gives up.
