Genghis Khan's conquest was made possible by his military brilliance. His brilliance was not just on the battlefield, but in other aspects of military operations such as communications and supplies and the coordination of his armies. He was innovative at gathering intelligence from any travelers. He was good at using both traditional Mongol military tactics and more "modern" tactics like the use of guns and of siege technology.
His consolidation of the territory he conquered was made possible by his political abilities. He created a system that allowed for central control of the empire, but also for a great deal of local autonomy. He allowed people he conquered to essentially rule themselves as long as they obeyed his overall authority. This made it easier to control and consolidate the territory conquered by his military brilliance.