One flaw in the co-equal treaties between the steppe-nomad Shanyu and Chinese emperor, T. Greer explains, was that the two sovereigns did not exercise equal amounts of control over their respective empires:
The Xiongnu confederation was an amalgamation of different tribal groups forged together by the Xiongnu ruling line. The Shanyu’s power was proportionate to the threats his people faced or the rewards he could promise them. If he tried to control the tribal autocracy with too tight a hand the inevitable consequence would be flight (to China or to other steppe confederations on the steppe) or open rebellion. If one of his subordinate leaders decided to campaign against the Han on his own accord there was little the Xiongnu could do stop him.
The other reason the Chanyu was willing to turn a blind eye to these attacks (and in occasion lead them himself) was that they strengthened the Xiongnu position at the negotiating table. “Almost each time a new pact was signed something was lost by the Han, and gained by the Hsiung-nu.” Far from being an instrument the Chinese used to sap the Xiongnu elite’s political cohesion (as Luttwak suggests), the treaty system was a way for the Xiongnu elite to extort the Han Dynasty.
That’s an awfully nice agrarian empire you’ve got there. I’d hate to see something happen to it.