Quote from above link:
"He also reports, “The Mongols destroyed mosques, palaces, grand buildings, hospitals, and libraries. The Mongols raided the House of Wisdom itself. The Tigris river ran black from the ink of the books that were thrown into the river, mixed with the blood of the slain.” The destruction the Mongols wreaked on the Muslim world was so great – it came close to wiping out Islamic civilization – that most Muslims of the time viewed it as a form of divine retribution for the sins they had committed."
It was a great era, when moslems were slaughtered by the 100s of thousands.