The problem was not enenmy. The problem was success. What happens when a leader dies? Either the leadership is divided or a struggle for power. If the Yelang are Hmong, looking at that kingdom there was no one to replace the dead king. Without a leader, all the other tribes went their separate way. Another thing is that the kingdom never went beyond an agrarian society. The allocation of resources was inefficient. There was no standing army to perfect Hmong art of war. No matter how fierce a people is in war, individuals fight against a group is out classed. The Romans proved that against the Gauls, Celtic, Vandals, etc.
In term of individuals working for the enemy, everyone has that problem. When Alexander fought the Persians, the fought against fellow Greeks. When Ceasar became emperor, he fought against fellow Romans. It is just a minor problem to overcome.