Specifically, on the start of the medieval period. When was it? Sure, it was after the Ancient or Classical Period, and before the Renaissance. But when did it begin? When does the age of classical civilization in Europe end? And what caused that to happen?
Traditionalists will immediately tell you that the Roman Empire fell in 476 AD, when Rome was sacked by Germanic barbarians. If you press them, they may edit that to include only the Western Roman Empire. But Henri Pirenne, an early 20th century Belgian historian argues for a much later date. I think his argument is pretty convincing, honestly. He claims that the Germans of the various tribes (with the exception of the Angles and Saxons who invaded Britannia) became Romanized rather than the conquered Romans becoming Germanized. Which seems to be true. Roman laws, language (Latin), political structure (autocracy/monarchy), religion (Catholic), economics, coinage (gold), agriculture, literature... all still the same. Only the guy(s) in charge changed.
Pirenne argues instead that the real change from a Mediterranean mindset in the majority of civilized Europe came after the Arab/Muslim invasions of the 7th and 8th centuries. This is what ended trade, changed population patterns, and re-centered western European civilization to the Carolingian Franks.
Its an interesting book. I'll discuss another book, on colonial American history, tomorrow or sometime soon, at any rate.