The Latin Christian siege of Constantinople in 1204 did irreversible damage. The Christian Romans lost to the Turks at Manzikert two centuries earlier but they might have survived the blow... if the West had not ransacked the capital of the Christian Roman world that had survived since Constantine's day.
"Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached." -Manuel II Paleologus