3. The Conquests of the Mongol Empire, spanning more than a century, resulted in the deaths of tens of millions in the course of constructing the largest contiguous empire in human history
Beginning in 1206, Genghis Khan, having united the Mongol people under a single banner for the first time in history, unleashed his new empire upon the world. Starting with the invasion of Western Xia, by 1209 the victorious Mongol hordes descended upon their ancient enemy: China. In 1211 the Mongols invaded Jin China, capturing and sacking the capital city of Yanjing. Pivoting to the West after the execution of his ambassadors by the Khwarezmid Empire, Genghis invaded Khwarezmia in 1919; taking just two years to conquer the Central Asian nation, the Mongols slaughtered at least 25% of the local population who suffered approximately 2,000,000 fatalities.
Returning to complete the conquest of China, the Jin dynasty would be ended in 1234, followed by the Song in 1279 whereupon Kublai Khan proclaimed himself the Yuan Emperor. In the interim years, the Mongols, under the leadership of Ögedei Khan, invaded Eastern Europe, defeating Bulgaria, Hungary, Poland, Russia, and Serbia, in addition to solidifying control over the Middle East after the sack of Baghdad in 1258. In total, across more than a century of warfare the Mongols are believed to have been directly responsible for the deaths of 30,000,000 to 40,000,000 people, in addition to displacing hundreds of millions and indirectly causing the calamitous Black Plague in Europe.