Tokyo bombings killed around 200,000 people, a small fraction of the 6 million who lived there. Genghis Khan would have had his soldiers kill most, if not all of the 6 million.
You mean like Roosevelt had his soliders kill most, if not all citizens of Hiroshima and Nagasaki? The objective of the two things is remarkably similar by the way - to demonstrate your might and make the enemy surrender out of fear.