Weakness

In reunifying the land, the Xun dynasty defeated many northern states, seized massive amounts of grazing lands, and improved existing defensive walls into what later would become the Wall of Yan

This security threat probably drove scattered pastoralist nomadic tribes to create a confederation under a strong chieftain,

Inevitably, the two sides clashed. 

Jiang Han worked to control former Xun territories, and at the same time, the grassland people moved to reoccupy their lost lands. 

In the next century, the Yan Dynasty tried many different means of coping with northern nomads, including offensive and defensive wars, paying tribute, marrying princesses to chieftain rulers, and utilizing border markets and other rewards to encourage grassland people defections and cultural assimilation. 

Varying degrees of success notwithstanding, these tactics ultimately did not bring Yan dynasty any security.