Many Muslim historians look upon the Mongols as looters and plunderers. They tell us that the Mongols were like the Goths and Vandals, destroying everything in their way with the only aim to loot established rich civilizations. These historians allege that the civilization of the Muslims at Baghdad was the richest in the 13th century. This is wrong, while Baghdad was a rich and well endowed city, the Caliphate owed its riches to the constant looting of Persia, Central Asia, North Africa, Spain which the Muslim armies had been indulging from the beginning of Islam in 630 C.E., till they were checked by Charles Martel in France in 732 C.E. and till their brutal march across Central Asia towards China was reversed with equal brutality by the Mongols from 1200 C.E.
The Man who almost destroyed Islam – Genghis Khan. Genghis Khan attacked the Turko-Persian Muslim Khwarazmian empire of Samarkand to avenge the attacks being launched by the Arab and Persian Muslims into Tartary (Central Asia). The Khwarazmıan empire was established by Turkmen and Kipchak Turks who had converted to Islam due to Persian and Arab depradations against them from the 7thcentury onwards. The Kipchak Turks had converted along with the Quarluqs after the Chinese defeat against the Arabs in the battle of the Talas river. By the 10th century, the nobility of the Kipchak Turks had interbred with the Persians and Arab Muslims and had established a large empire over Central Asia. This Turko-Persian Muslim Khwarazmian empire had carried on the Muslim tradition of attacking neighboring non-Muslim peoples. Their depradations against other non-converted Turks and Mongols from the 8th century onwards, gradually built a simmerring resentment among the non-Muslim Turks and Mongols against the Turko-Persian Muslim Khwarazmian empire
Genghis Khan’s intention was not primarily to loot, but to destroy the enemy. Had the Mongols been motivated purely by intentions of looting the Caliphate (which ironically was itself a center where loot was collected and stored by the Muslims), the Mongols need not have traversed some four thousand miles from their homeland in Mongolia, to reach Baghdad, they could have as well attacked nearby Japan and Korea which were hardly a few hundred miles from their homeland and were more rich and endowed than Baghdad.
More details here
http://www.historyofjihad.org/mongolia.html
and on the historical context of Muslim fatwas against Mongols here:
https://sites.google.com/site/jhoover36 ... he-mongols