I largely concur. From just about everything I've ever read, the 4th Crusade was simply too devastating a blow for the Empire to recover from. While earlier events like Manzikert obviously hurt the Byzantines badly as well -- to say nothing of their general internal/political weakness(es) which had been dogging them for centuries -- I think the Crusaders' sacking of the city was what really marked the beginning of the end.
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