From 1219 to 1221, under the Khwarazmian Empire, Iran suffered a devastating invasion by the Mongol Empire army of Genghis Khan. According to Steven R. Ward, "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." Most modern historians either outright dismiss or are highly skeptical of such statistics of colossal magnitude pertaining the Mongol onslaught on the Khwarazmian empire, mainland Iran and other Muslim regions and deem them to be exaggerations by Muslim chronichlers of that era (whose recordings were naturally of an anti-Mongol bent). Indeed, as for as the Iranian plateau was concerned the bulk of the Mongol onslaught and battles were in the north-east of what is modern day Iran. Such as the cities of Tus and Nishapur.