From A History of Iran: Empire of the Mind, by Michael Axworthy (Basic Books, 2016), Kindle Loc. 2008-32:
Khorasan suffered terribly again as the Mongols moved in to punish those who continued to resist, and to set up their occupation regime. In Tus, which they made their base, the Mongols initially found only fifty houses still standing. The golden age of Khorasan was over, and in some parts of the region agriculture never really recovered. Where there had been towns and irrigated fields, the war horses of the conquerors and their confederates now were turned out to graze. Wide expanses of Iran reverted to nomad pastoralism, but these nomads were more dangerous, ruthless mounted warriors of a different kind. Peasants were subjected to taxes that were ruinously high and were collected after the fashion of a military campaign. Many fled the land or were forced into slavery, while those artisan city dwellers who had survived the massacres were forced to labor in workhouses for their conquerors. Minorities suffered, too. In the 1280s a Jew was appointed as vizier by the Mongols, but his appointment grew unpopular, he fell from office, and Jews were attacked by Muslims in the cities, establishing a dismal pattern for later centuries: “[They] fell upon the Jews in every city of the empire, to wreak their vengeance upon them for the degradation which they had suffered from the Mongols.” It was a grim time indeed. Khorasan was more affected than other parts, but the general collapse of the economy hit the entire region.
The Mongols, who made Tabriz their capital, spent the next few decades consolidating their conquests and destroying the Ismaili Assassins in the Alborz mountains, just as the Seljuks had tried and failed to do for many years before 1220. Some smaller rulers who had submitted to the Mongols were allowed to continue as vassals, and in the west the rump of the Seljuk Empire survived in Anatolia on the same basis as the Sultanate of Rum. In 1258 the Mongols took Baghdad. They killed the last Abbasid caliph by wrapping him in a carpet and trampling him to death with horses.
Yet within a few decades, astoundingly, or perhaps predictably, the Persian class of scholars and administrators had pulled off their trick of conquering the conquerors—for the third time. Before long they made themselves indispensable. A Shi‘a astrologer, Naser od-Din Tusi, captured by the Mongols at the end of the campaign against the Ismailis, had taken service with the Mongol prince Hulagu, and served as his adviser in the campaign against Baghdad. Naser od-Din Tusi then set up an astronomical observatory for Hulagu in Azerbaijan. One member of the Persian Juvayni family became governor of Baghdad and wrote the history of the Mongols; another became the vizier of a later Mongol Il-Khan, or king. Within a couple of generations Persian officials were as firmly in place at the court of the Il-Khans as they had been with the Seljuks, the Ghaznavids, and earlier dynasties. The Mongols initially retained their paganism, but in 1295 their Buddhist ruler converted to Islam along with his army. In 1316 his son Oljeitu died and was buried in a mausoleum that still stands in Soltaniyeh—one of the grandest monuments of Iranian Islamic architecture and a monument also to the resilience and assimilating power of Iranian culture.