The nomads to the north were incredibly militaristic, almost every male nomad had lived his entire life riding and shooting bows from horseback. Raiding was common across different nomadic communities as well as hunting being a chief source of food so you got a group of people who without needing to be part of a trained army were some of the most effective soldiers in the world. The danger of horse archery before the widespread proliferation of guns should never be underestimated. Horse archers could deal a huge amount of damage to the enemy via archery without having to close with the enemy and unlike foot based archers had extremely high mobility, allowing them to escape from poor situations as well as allowing them to surprise enemy armies and conduct extensive flanking operations. For the Mongol army at least, in his "The Mongol Art of War", Timothy May states that discounting chinese and persian siege engineers, a mongol army would be made up entirely of cavalry, about 6/10 horse archers and about 4/10 lancers to rout the enemy once they had been sufficiently disorganized and weakened by the archers. This is why despite their vastly fewer numbers the nomads to the north of China had been always able to hold their own against the chinese armies and at certain points in history able to partially or even fully conquer China. I cannot say for the peoples to the south of China.