One estimate is that approximately 10% of the contemporary global population, amounting to some 37.75–60 million people, was killed either during or immediately after the Mongols' military campaigns.

[8] As such, the Mongol Empire, which remains the largest contiguous polity to ever have existed, is regarded as having perpetrated some of the deadliest acts of mass killing in human history.

For instance, Goryeo was assessed at 10,000 otter skins, 20,000 horses, 10,000 bolts of silk, clothing for soldiers, and large numbers of children and artisans as slaves.

From the perspective of modern theories of international relations, Quester suggested, "Perhaps terrorism produced a fear that immobilised and incapacitated the forces that would have resisted.

[4] Ancient sources described Genghis Khan's conquests as wholesale destruction on an unprecedented scale in certain geographical regions, causing great demographic changes in Asia.

[16] Mustawfi found Merv city still in ruins in the midth of 14th century with the sands of Qara qum encroaching on the arable lands of the oasis.

[16] Persian philosopher Najm al-din Razi, who was contemporary of Genghis Khan and Subutai, wrote that Mongol invaders killed 700,000 people in his hometown Ray.

Other historians, like William McNeill and David Morgan, argue that the Black Death, spread by the Mongols, was the main factor behind the demographic decline in that period.

[29] Mongol campaigns in Northern China, Central Asia, Eastern Europe and the Middle East caused extensive destruction, but there are no exact figures available for that time.

The cities of Balkh, Bamiyan, Herat, Kiev, Baghdad, Nishapur, Merv, Konye-Urgench, Lahore, Ryazan, Chernigov, Vladimir and Samarkand suffered serious devastation by the Mongol armies.

[30][31] For example, there is a noticeable lack of Chinese literature from the Jin dynasty, predating the Mongol conquest, and in the Siege of Baghdad (1258), libraries, books, literature, and hospitals were burned: some of the books were thrown into the river in quantities sufficient to turn the Tigris black with ink for several months, according to legend;[32][33][34][35] further, "in one week, libraries and their treasures that had been accumulated over hundreds of years were burned or otherwise destroyed.

"[36] Genghis Khan was largely tolerant of multiple religions, but there are many cases of him and other Mongols engaging in religious war even if the populations were obedient.

[39] According to a study by the Carnegie Institution for Science's Department of Global Energy, the annihilation of so many human beings and cities under Genghis Khan may have scrubbed as much as 700 million tonnes of carbon from the atmosphere by allowing forests to regrow on previously populated and cultivated land.