Ghengis tried to set up a peaceful trading empire after conquering China. He didn't go back on the warpath and start heading for Europe until the idiot Persians sent his ambassador's head back to the "weak and distant barbarian" Khan in a bag. Unfortunately, the ambassador was a friend of the Khan.
We all know what happened next.
Didn't they also shave off the ambassador's beard or something?
I think I read someplace that such an act was a sign of terrible disrespect.