He looked to the west to build on his conquests, not through war, but through trade. He sent ambassadors to Persia, and established a network of routes linked by staging posts, 40 kilometers apart. A messenger could travel 200 kilometers in a single day. It was a medieval pony express.

In the summer of 1218, one messenger sent back to Genghis Khan from Persia carried a package, a package that would change the course of history. It was the head of Genghis Khan's ambassador. The sultan had fundamentally misjudged Genghis Khan's character. Genghis Khan sent an army 200,000 strong to invade Persia. It was a campaign of extreme savagery.

"I was not the instigator of these tribulations. God grant me the strength to exact vengeance."

On his order, every Persian town that did not submit was burned to the ground. When they had finished, over a million men, women and children were dead.

After the conquest of Persia, Genghis Khan ordered a small army to see how far west they could penetrate before they were stopped, and they weren't stopped at all. This was the Mongols' first raid into Europe. It would not be their last.

His empire was now four times the size of Alexander the Great's(亚历山大大帝), and twice the size of the Roman Empire.

staging post: a place at which people or vehicles regularly stop during a journey

pony express: postal system using horseback riders and trains to deliver the mail from the Midwestern United States to the west Coast in the mid 1800's

sultan: a Muslim sovereign