Genghis was worse than Hitler
Genghis declared war, and took many rich cities with frightful bloodshed. The Mongols swept across the Iranian interior, leaving a trail of destruction in their wake. Whole cities were put to the torch and mass killings of women and children as well as fighting men were common.
Mongols killed many civilians. Just in Merv and Urgench(Gorganj) about a million civilians were slaughtered.
During 1220–21 Bukhara, Samarkand, Herat, Tus, and Neyshabur were razed, and the whole populations were slaughtered. the total population of the Persian Empire may have dropped from 2,500,000 to 250,000 as a result of mass extermination by Mongols and subsequently famine.The results of the Mongol invasion for the Iranian economy were disastrous. The well-developed networks of qanat irrigation systems that had previously made possible a largely continuous pattern of habitation across large areas of Iran were laid to waste, leaving a series of isolated oasis towns in its place. Furthermore, since the population had been decimated, Iran was left without the workforce required to recover itself.
The library of Alamut was put to fire, denying subsequent scholars the knowledge that could have unlocked the secrets of the Ismailis and the schools and libraries founded by Nezam al-Molk were also destroyed. It is said that the schools at Nishapur burned for months before all of its treasures were finally consumed.
The Mongols were sometimes called the Tatars, which is actually a corruption of the Chinese term for one of the Mongol peoples. In Roman mythology, however, Tartarus was the Roman equivalent of Hell. There were famous to make towers out of Peoples head.
The Mongol subjugation of Russia was brutal and humiliating and contributed greatly to that sense of tragedy that so deeply imbues Russian culture and art.
Like the Seljuks before them, the Mongols were very open to the cultural influences of the civilisations that they had conquered. They were practical enough to admit Persian scholars, physicians, jurists and soldiers into circles of the highest rank.