Life In Iran

In 1218, the eastern Khwarazmid provinces of Transoxiana and Khorasan suffered a devastating invasion by Genghis Khan. During this period more than half of Iran's population was killed, turning the streets of Persian cities such as Nishapur into "rivers of blood", as the severed heads of men, women, and children were "neatly stacked into carefully constructed pyramids around which the carcasses of the city's dogs and cats were placed".
According to Steven R. Ward, "Overall, the Mongol violence and depredations killed up to three-fourths of the population of the Iranian plateau, possibly 10 to 15 million people. Some historians have estimated that Iran's population did not again reach its pre-Mongol levels until the mid-20th century." In a letter to King Louis IX of France, Holaku, one of the Genghis Khan's grandsons, took sole responsibility for 200,000 deaths in his raids of Iran and the Caliphate. He was followed by yet another conqueror, Tamerlane, who established his capital in Samarkand. The waves of devastation prevented many cities such as Nishapur from reaching their pre-invasion population levels until the 20th century, eight centuries later.
In 1387, Tamerlane avenged a revolt in Isfahan by massacring 70,000 people. But both Hulagu, Tamerlane, and their successors soon came to adopt the ways and customs of that which they had conquered, choosing to surround themselves with a culture that was distinctively Persian. The mid-14th-century Black Death killed about 30% of the country's population.
Iran was gradually Islamized after the collapse of the Sassanid Empire; however, it was not Arabized. Iranian culture re-emerged with a separate and distinctive character and made an immense contribution to the Islamic civilization. When Islam came through Iran, what developed was an Iranian Islam or Persian Islam rather than the original Arab Islam, and this new Islam is sometimes referred to by scholars as Islam-i Ajam (Persian Islam).