Genghis Khan's wars are estimated to have killed 40,000,000 people.
Genghis Khan was convinced of the superiority of the nomadic lifestyle and did not understand the importance of trade, cities, administration, etc.
The early years of his wars. For him, razing a city was no more difficult than cutting down a tree. He thus used terror to wage war. People had every incentive to surrender, otherwise the Khan would raze the city once it was captured.
For settled populations, razing a wealthy, trading city was incomprehensible, as the aim of the war would have been to seize the city. But for nomads, wealth is not measured in land but in horses. Ultimately, northern China and present-day Iran were almost entirely destroyed.
Dozens of cities, including some of the richest in the world, were wiped off the map. Some were never rebuilt. The city of Merv, for example, was known as one of the most prosperous cities in the world. Since the Mongol invasion, nothing remains of it.