What would be the effects on society (possibly until the 21th century) if during the Fourth Crusade the Romans manage to repel the crusaders and thwart the Sack of Constantinople?
Delayed Turkish advance into Europe...?
Btw I have an interesting idea, what if the Emperor in Constantinople allied himself with the Mongols when they invade Europe?
(when the Mongols came in OTL, there was no "Byzantine Empire" per se...only Trebizond, Morea, and Epirus...)
What would be the effects on society (possibly until the 21th century) if during the Fourth Crusade the Romans manage to repel the crusaders and thwart the Sack of Constantinople?
For conditions in Asia Minor at this period see: The Decline of Medieval Hellenism in Asia Minor, by Speros Vryonis.Personally I don't see how no 4th Crusade would weaken Byzantine postion in Anatolia. More resources, less wars and so on. Not mentioning that Mongols most likely show as per schedule, which means that in late XIII C. both Rum and Bulgaria are severly weakened, which would most likely allow restoration of Byzantine Empire in large parts of Anatolia and Balkan.
The curious butterfly effect would be on Ukraine. If things go more-or-less as OTL (Mongols, expansion of Lithuania) by XVI-XVII C. there'd be an increasingly important wheat trade route - Ukraine - Byzantium - South-Western Europe.