From here.There are many apocryphal quotes attributed to Genghis Khan. And there’s a reason for that — in a single generation he led an obscure group of Mongolian tribes to conquer most of the known world. His armies, and those of his descendants, ravaged lands as distant as Hungary, Iran and China. After the great wars, though, came great peace — the Pax Mongolica. But the scale of death and destruction were such that in the wake of the Mongol conquests great forests grew back from previously cultivated land, changing the very ecosystem of the planet.