I think Mongols is great and I suggest it should be an offensive nations or rather to be defensive nations based on it's available resources esp food?
I concur with Brown Bear, Mongols was short lived due to they overpowered and had one of the biggest land empire or they had conquered, until they had insufficient armies to defend the cities before they conquer the next world. From the history I readed on Chinese books, Gengkhis Khan or Temujin is a smart general who forced out his live from being a slave in his young age and became a leader at early 20s+.
He quickly formed a group or armies and alliances and then they raided the cities without any mercy. His generals even told him, to retain the opposition's generals or leaders and forced them to be his followers, but most of the late wars that he became stronger, Khan just wanted everyone to be killed and if not mistaken, the number of troops he had killed was equivalent to 1/3 of the current world population.