Your arrogance causes Genghis Khan to turn his full attention towards your lands. He invades with 150,000 men, and within 2 years your empire is wiped off the face of the earth.
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Chapters 10 and 11 in Genghis Khan: The Man Who Conquered the World by Frank McLynn are probably the most detailed accounts of the ensuing decimation that I came across. Highlights include detailed analysis of the military strategies that Mongols used, as well as discussion of their (often overlooked) espoinage.