Here's a question I've tried to get at in some form or fashion with different threads in the past, but I figured I'd ask it more directly. After the fall of the Western Roman Empire, much of its former territory was divided between kingdoms established by different Germanic tribes. Important examples include the Ostrogoths and Lombards in Italy, the Visigoths in Gaul and Hispania, the Franks in Gaul, the Vandals in North Africa. However, with a few exceptions like the Anglo-Saxons in Britain, these area did not really Germanize in the long run. Romance languages still predominate in Southwestern Europe, and North Africa is part of the Arab world. So, what would it take for most or all of the former Western Roman Empire to Germanize in the long run (linguistically, culturally, the whole nine yards) in the same way that the Arab language and Islamic religion came to predominate in many areas conquered by the early Muslim conquerors? I guess the first step would be to prevent Justinian's partial reconquest of the west by the Eastern Roman Empire (maybe distractions in the Balkans or from the Persian Empire?), and also any other conquests from outside groups (for instance, nothing like the Islamic conquests, which would be butterflied away in such a scenario anyway), but is that sufficient, or merely necessary?