The Pax Mongolica came about because of the stability of Genghis Khan's huge empire. The Pax Mongolica basically means... O An era of warfare
O An era of peace
O An era of chaos
O An era of learning

From what I know, neither the United States nor the Soviet Union was responsible. Because they had tensions between each other. The United States was trying to stop communism while the Soviet Union was trying to spread communism. I hope this has helped you.