In the early 13th century, the Khwarazmian dynasty was governed by Shah Ala ad-Din Muhammad. Genghis Khan saw the potential advantage in Khwarezmia as a commercial trading partner using the Silk Road, and he initially sent a 500-man caravan to establish official trade ties with the empire. However, Inalchuq, the governor of the Khwarezmian city of Otrar, attacked the caravan that came from Mongolia, claiming that the caravan contained spies and therefore was a conspiracy against Khwarezmia. The situation became further complicated because the governor later refused to make repayments for the looting of the caravans and handing over the perpetrators. Genghis Khan then sent again a second group of three ambassadors (two Mongols and a Muslim) to meet the Shah himself instead of the governor Inalchuq. The Shah had all the men shaved and the Muslim beheaded and sent his head back with the two remaining ambassadors. This was seen as an affront and insult to Genghis Khan. Outraged, Genghis Khan planned one of his largest invasion campaigns by organizing together around 200,000 soldiers (20 tumens), his most capable generals and some of his sons. He left a commander and number of troops in China, designated his successors to be his family members and likely appointed Ögedei to be his immediate successor and then went out to Khwarezmia.
The Mongol army under Genghis Khan, generals and his sons crossed the Tien Shan mountains by entering the area controlled by the Khwarezmian Empire. After compiling intelligence from many sources Genghis Khan carefully prepared his army, which was divided into three groups. His son Jochi led the first division into the northeast of Khwarezmia. The second division under Jebe marched secretly to the southeast part of Khwarzemia to form, with the first division, a pincer attack on Samarkand. The third division under Genghis Khan and Tolui marched to the northwest and attacked Khwarzemia from that direction.
The Shah’s army was split by diverse internal disquisitions and by the Shah’s decision to divide his army into small groups concentrated in various cities. This fragmentation was decisive in Khwarezmia’s defeats, as it allowed the Mongols, although exhausted from the long journey, to immediately set about defeating small fractions of the Khwarzemi forces instead of facing a unified defense. The Mongol army quickly seized the town of Otrar, relying on superior strategy and tactics. Genghis Khan ordered the wholesale massacre of many of the civilians, enslaved the rest of the population and executed Inalchuq by pouring molten silver into his ears and eyes, as retribution for his actions. Near the end of the battle the Shah fled rather than surrender. Genghis Khan ordered Subutai and Jebe to hunt him down, giving them 20,000 men and two years to do this. The Shah died under mysterious circumstances on a small island within his empire.
The Mongols’ conquest, even by their own standards, was brutal. After the capital Samarkand fell, the capital was moved to Bukhara by the remaining men, while Genghis Khan ordered two of his generals and their forces to completely destroy the remnants of the Khwarezmid Empire, including not only royal buildings, but entire towns, populations, and even vast swaths of farmland. According to legend, Genghis Khan even went so far as to divert a river through the Khwarezmid emperor’s birthplace, erasing it from the map.
The Mongols attacked Samarkand using African prisoners as body shields. After several days only a few remaining soldiers, die-hard supporters of the Shah, held out in the citadel. After the fortress fell, Genghis supposedly reneged on his surrender terms and executed every soldier that had taken arms against him at Samarkand. The people of Samarkand were ordered to evacuate and assemble in a plain outside the city, where they were killed and pyramids of severed heads raised as a symbol of victory. Ata-Malik Juvayni, a high official in the service of the Mongol empire, wrote that in Termez, on the Oxus, “all the people, both men and women, were driven out onto the plain, and divided in accordance with their usual custom, then they were all slain”.
The city of Bukhara was not heavily fortified, with a moat and a single wall, and the citadel typical of Khwarezmi cities. The city leaders opened the gates to the Mongols, though a unit of Turkish defenders held the city’s citadel for another twelve days. Survivors from the citadel were executed, artisans and craftsmen were sent back to Mongolia, young men who had not fought were drafted into the Mongolian army and the rest of the population was sent into slavery. As the Mongol soldiers looted the city, a fire broke out, razing most of the city to the ground. Genghis Khan had the city’s surviving population assemble in the main mosque of the town, where he declared that he was the flail of God, sent to punish them for their sins.
Meanwhile, the wealthy trading city of Urgench was still in the hands of Khwarezmian forces. The assault on Urgench proved to be the most difficult battle of the Mongol invasion and the city fell only after the defenders put up a stout defense, fighting block for block. Mongolian casualties were higher than normal, due to the unaccustomed difficulty of adapting Mongolian tactics to city fighting.
As usual, the artisans were sent back to Mongolia, young women and children were given to the Mongol soldiers as slaves, and the rest of the population was massacred. The Persian scholar Juvayni states that 50,000 Mongol soldiers were given the task of executing twenty-four Urgench citizens each, which would mean that 1.2 million people were killed. The sacking of Urgench is considered one of the bloodiest massacres in human history.
In the meantime, Genghis Khan selected his third son Ögedei as his successor before his army set out, and specified that subsequent Khans should be his direct descendants. Genghis Khan also left Muqali, one of his most trusted generals, as the supreme commander of all Mongol forces in Jin China while he was out battling the Khwarezmid Empire to the west.