The Mongolian Empire was the best fighting force for their time. People would even surrender to them before the battle started! They started in the 1200's as one of the most brutal military forces. They moved in divisions of 10,000 men but only moved about 5 miles a day. However, scout forces could move as much as 100 miles a day and provide information to the rest of the army. For a long time the Mongols were separated into different clans lead by a khan, until a person named Genghis Khan, meaning universal ruler, took over and united them all. They eventually conquered China but chose to stay away from them and didn't allow Mongols and Chinese to marry. They burdened the Chinese with heavy taxes and made it so only non Chinese could hold power and even had some outsiders have some power in China.
The Yuan Dynasty started in 1260 when Kublai Khan became the Great Khan. He spent time trying to finish the China conquest and succeeded in 1279 when the last Song ruler fell. He moved his capital to China and tried to get them to like him, but they say him as rude and uncivilized. He did vastly increase their trading and even built better boats for travel. However, the Dynasty got a lot weaker during the end of Kublai Khan's reign and with a lot of battles lost they were eventually taken over.