<Li> Western researcher R.J. Rummel estimated that 30 million people were killed under the rule of the Mongol Empire . Some estimates go as high as 80 million killed, with 50 million deaths being the middle ground . The population of China fell by half in fifty years of Mongol rule . Before the Mongol invasion, the territories of the Chinese dynasties reportedly had approximately 120 million inhabitants; after the conquest was completed in 1279, the 1300 census reported roughly 60 million people . While it is tempting to attribute this major decline solely to Mongol ferocity, scholars today have mixed opinions regarding this subject . Scholars such as Frederick W. Mote argue that the wide drop in numbers reflects an administrative failure to record rather than a de facto decrease, whilst others such as Timothy Brook argue that the Mongols reduced much of the south Chinese population, and very debatably the Han Chinese population, to an invisible status through cancellation of the right to passports and denial of the right to direct land ownership . This meant that the Chinese had to depend on and be cared for chiefly by Mongols and Tartars, which also involved recruitment into the Mongol army . Other historians such as William McNeill and David Morgan argue that the bubonic plague was the main factor behind the demographic decline during this period . </Li> <Li> The Islamic world was subject to massive changes as a result of Mongol invasions . The population of the Iranian plateau suffered from widespread disease and famine, resulting in the deaths of up to three - quarters of its population, possibly 10 to 15 million people . Historian Steven Ward estimates that Iran's population did not reach its pre-Mongol levels again until the mid-20th century . </Li> <Li> David Nicole states in The Mongol Warlords, "terror and mass extermination of anyone opposing them was a well tested Mongol tactic ." About half of the Russian population may have died during the invasion . However, Colin McEvedy in Atlas of World Population History, 1978 estimates the population of Russia - in - Europe dropped from 7.5 million prior to the invasion to 7 million afterwards . Historians estimate that up to half of Hungary's two million population were victims of the Mongol invasion . Historian Andrea Peto says that Rogerius, an eyewitness, said "the Mongols killed everybody regardless of gender or age" and that "the Mongols especially' found pleasure' in humiliating women ." </Li> <Ul> <Li> One of the more successful tactics employed by the Mongols was to wipe out urban populations that refused to surrender . During the Mongol invasion of Rus', almost all major cities were destroyed . If they chose to submit, the people were generally spared, though this was not guaranteed . For example, the city of Hamadan in modern - day Iran was destroyed and every man, woman, and child executed by Mongol general Subadai, after surrendering to him but failing to have enough provisions for his Mongol scouting force . Several days after the initial razing of the city, Subadai sent a force back to the burning ruins and the site of the massacre to kill any inhabitants of the city who had been away at the time of the initial slaughter and had returned in the meantime . Mongolian armies made use of local peoples and their soldiers, often incorporating them into their armies . Prisoners of war sometimes were given the choice between death and becoming part of the Mongol army to aid in future conquests . In addition to intimidation tactics, the rapid expansion of the empire was facilitated by military hardiness (especially during bitterly cold winters), military skill, meritocracy, and discipline . </Li> </Ul>

What led to the downfall of the mongol empire