The fourth Crusade’s sacking of Constantinople was the biggest act of theft in history, and fatally weakened the Empire. Once the city fell to those bastard turkmen, europe realized a bit late how stupid they had been not to help.
In hindsight, we can see that it was inevitable; demographics favor those who have multiple wives. Would Europe today (with its large Muslim populations) intervene if Constantinople was being beseiged now? Looking at the Muslim purge of Christians from Iraq and Syria, I think we can safely assume Europe wouldn’t lift a finger.