The wikipedia page on zoroastrians in Iran says that when Mongols invaded Iran, they mostly directed their attention towards muslims, because being pagans and buddhists, they hated islam. How true is this? Where does this animosity stem from? Why would Mongols differentiate between islam and other religions?
I should start by clarifying something. The Mongol Uls ("confederation") that Temujin was born into was primarily shamanistic. By the time he became Genghis Khan, the Mongol Uls had conquered and absorbed the four other major confederations to form the Mongol Khanate. The people of this khanate were a mix of shamanists, Nestorian Christians, Buddhists, Taoists, Muslims, and possibly Manichaeists. The ruling class of the khanate was quite tolerant of religions as long as the people paid their dues
In fact, one of the early westward campaigns liberated Muslim populations from the brutal Buddhist-Christian oppression of Kushluk. He was a Naiman (an opposing confederation that followed Nestorian Christianity) who had fled to the allied Western Liao Empire (whose ruling class was largely Buddhist), where he then usurped the throne and oppressed the Muslim people. Genghis Khan's army was welcome as liberators
The issue with Iranians lay with its current dynasty, Khwarezm. In particular, one governor executed a caravan of Mongol merchants. Or spies. It depends on the perspective. Of course, maybe there was some miscommunication. I personally believe that they were merchants because Genghis Khan was currently engaged against the Jurchens to the east, and it would not make sense to turn his attention to Khwarezm in the west. But whatever the truth was, the envoy he sent was executed by the Shah
As a result, this became a war of revenge, no different from the one waged in the east against the Jurchens. The difference is that Khwarezm happened to have many Muslims. And unlike in the east, Genghis Khan could not find any local populations that would join him willingly, such as the Han Chinese and Khitans who held grudges against the Jurchen conquerors from Manchuria. As a result, the famous "surrender or massacre" policy reached its zenith. This policy would be used again and again throughout Islamic lands against polities like the Hashashin and the Caliphate to strike fear and destroy their political organizations. Many Muslims suffered and died as a result of Genghis Khan's policy, but not due to a specifically anti-Islamic intent, at least during his campaign. His grandson Hulegu may have been anti-Islamic, but it seems to me that he was more pro-Mongol in Islamic lands than specifically anti-Islamic
Either way, it is undeniable that the Mongols were quite friendly towards Muslims they liberated and Muslims who paid their dues as citizens in the khanate. But regardless of religion, all enemies were subjected to brutal policies. Tragically, many of the worst atrocities have been connected to Islamic lands and the end of the Golden Age of Islam
Chinese Sources
English Sources