10 February 1258

The Mongols destroy Baghdad.

This is an interesting event, because if Muslims want to blame the West for all their problems, they might want to blame the Mongols. In 1258 the Mongols swept through Iraq and destroyed the Abbasid Caliphate, which had ruled most of the Muslim world for 500 years. Despite a lot of problems, the Abbasids had kept the large empire established by the early Arab conquests together and incorporated many more ethnic groups, including the Persians. They also absorbed Hellenistic culture and promoted rational inquiry and learning. Perhaps the most famous Caliph, Harun al-Rashid, was an Abbasid. Even though their empire wasn't as monolithic as they probably wanted to think, after the Mongols destroyed one of the great cities of the Middle Ages, the Muslim world became even more fractured and insular. This event separates the Golden Age of Islam from what can be seen as a slow decline, at least among the Arab portions of the Islamic empire. Islam recovered under the Ottomans, but it took almost 200 years. More on the Abbasids here.