The Mongol Empire proved to be much too large for Genghis Khan's remaining sons to administer. This is why the nation itself split off into separate controlled areas by which each of his sons had control off. Prior to this, sibling feuds had become a common thing between them anyway, so it's not hard to believe that anarchy took place in between the time of the Khans death, and before they split during very late years this empire existed. These regions held out on their own for a limited amount of time before falling in a matter of years that were rather petty compared to that of what Genghis Khan formed. The Yuan dynasty was one of the new growing powers within China, really a signal of death to where the Mongol Empire would end, seeing as the Chinese already began seizing smaller parts of southern Mongolia, cutting them off more.