It was, as your link pointed out, the establishment of the Latin kingdoms in Greece and Asia Minor which resulted from the fall of Constantinople to the Crusaders. However, the city could have remained intact and the Latin kingdoms would still have been established.
I don't think I would mind so much if it weren't for the fact that all the art was destroyed. If the Crusaders had been a bit more aesthetically inclined and the treasures had just been stolen, I wouldn't care (being that I am not a Byzantine). The problem was that so much of it was just destroyed...
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