The entire Mongol Empire, at its peak, covered some 12.7 million square miles, which is 22% of all the land area on Earth. The tactics that enabled such conquests can be traced primarily to Genghis, the empire’s founder and first Khan. His birth name was Borjigin Temujin, and he devised a versatile attacking style, that of missile cavalries: his best archers were not trained merely to shoot, but to shoot accurately while riding horses at full gallop. They could even shoot accurately directly behind the horse at full gallop. No infantry force in the world at that time could have withstood such soldiers, and all the nations the Mongols invaded were overwhelmed very quickly.
Genghis’s legacy has been cemented by his conquest of Khwarezmia, which is most of modern Iran, along with parts of Afghanistan, Turkmenistan, Tajikistan, Uzbekistan, Kyrgyzstan and Kazakhstan. Genghis originally respected the leader, Ala ad-Din Muhammad II, as another conqueror, but when Genghis sent emissaries to strike up commerce with Ala ad-Din, the latter killed the diplomats and sent the rest back with shaved heads as an insult. First rule of Genghis Khan: don’t insult Genghis Khan.
He invaded Khwarezmia with 200,000 men, as many as half of these mounted archers, and split his army into smaller forces designed to conquer more territory faster. In military schools, this is always advised against, but Genghis’s scouts indicated that Ala ad-din was waiting with stronghold defenses, which suited Genghis’s desire for maneuvering room. His armies surrounded the walled cities of Samarkand, Urgench and Bukhara, and utterly destroyed them, one after another. On the third day of siege at Burkhara, the Turkish generals inside decided that they did not have the food and water to outlast Genghis, and so sallied a force of 20,000 cavalrymen and infantrymen, who attacked in the open steppe outside the city. Genghis’s army slaughtered them, to the last man.
Then he finished the siege within another 2 weeks, killed the Turkish soldiers who survived, sent the rest of the population’s youth into slavery, and executed everyone else, men and women deemed inefficient for labor. Seeing that the Turkish attempt to free itself from siege failed so well, Genghis next besieged Samarkand, whose garrison sent 50,000 veteran troops against Genghis’s army when it pretended to withdraw piecemeal. This was a simple plot that worked magnificently. His men retaliated, flanked on both sides, enveloped, and shot the Turks down in a massive pile of human and horse carcasses. He saw no need to preserve their horses since his did not seem to be at risk. Ala ad-Din arrived with a relief force of several tens of thousands, but could not approach because of Genghis’s mounted archers. The other 50,000 or so defenders of the city were executed to the last man, as was every single civilian, whose heads were arranged into a giant pyramid outside the walls.
Urgench was not so easy to attack, since it was built on swamp land around Amu Darya River. Genghis sent his men in without fear, and they lost significantly more men than usual due to the urban street fighting. The high end estimate of Turkish deaths, both civilian and military, in Urgench is 1,200,000, but much more plausible is 250,000 to 500,000. The rest were enslaved. This was one of the bloodiest genocides in history.