So we are learning a bit about world history in my class and I was researching them online but I just can't fathom how these numbers are accurate.
###They didn't face all 30 million at once.
Just the campaigns of Genghis Khan spanned over twenty years (1206-1227). So even if those 30,000,000 died over that span, that's "only" 1.4m/year. 100,000 warriors would have, on average, needed to kill 15 people each per year - essentially only a bit more than one person per month.
###Most of the people killed were women, children, and the elderly.
The number of dead include women and children, most of whom were not combatants. Men comprise roughly only half any given population, and most children and elderly aren't fit for combat. So the effective fighting force for a given population is around 30% at most, in my estimation.
###The Mongols fought many unskilled fighters
Some larger states and cities had fit, ready fighting forces, but much of the countryside towns and villages were protected by men who had never seen or been trained for combat. Against the onslaught of combat veterans in the Mongol horde, they would have been easy prey.
###The Mongols fought dirty According to this paper at the CDC , when the city of Kaffa revolted and settled in for a prolonged siege, the Mongols took cadavers of their own people infected with the Bubonic Plague and catapulted them over the city walls, causing a plague outbreak in the city.
[Wheelis M. Biological Warfare at the 1346 Siege of Caffa. Emerg Infect Dis. 2002;8(9):971-975. https://dx.doi.org/10.3201/eid0809.010536],
[Horrox R, ed. The Black Death. Manchester: Manchester University Press; 1994. p. 14–26.]
Sorry for the overall lack of citations - did this in a bit of a rush. Please feel free to correct any errors or omissions.
On a related note, how many of the deaths were due to direct mongol action (such as executing an entire city, or deaths in battle) versus indirect action (such as starvation occurring as a result of mongol campaigns)?