Last year I read a book called "Genghis Khan and the making of the modern world" (on Amazon here -> http://amzn.com/0609610627). While some parts were a bit gross and there were definitely some slower portions of the book, it was a very good read. Purported facts like portions of his (and his successors') DNA appear in 0.5-2% of the world's men (depending on who generates the figure), his regime established (or at least vastly improved) the modern concepts of credit, tax exemption for religions, religious freedom (to an extent), trade routes still in use today, etc etc etc only fueled his legendary status whether he was hated or loved by everybody under his command. He is one of the best examples of rags to riches (in his case leather clothes to nicer leather clothes) in the history of humanity.
Now its emerging that by the sheer volume of people his wars killed, it actually cooled the earth!
From the article:
The Mongol leader, who established a vast empire between the 13th and 14th centuries, helped remove nearly 700million tons of carbon from the atmosphere, claims a new study.
The deaths of 40million people meant that large areas of cultivated land grew thick once again with trees, which absorb carbon dioxide from the atmosphere.
And, although his methods may be difficult for environmentalists to accept, ecologists believe it may be the first ever case of successful manmade global cooling.
The story of Genghis Khan is nothing if not incredible, do yourself a favor and read the book, I'll even lend it to you if you're nearby and I know you
Source -> http://www.dailymail.co.uk/sciencetech/article-1350272/Genghis-Khan-killed-people-forests-grew-carbon-levels-dropped.html