Who were the Xiongnu--from wikipedia:

Significance

The Xiongnu confederation was unusually long-lived for a steppe empire. The purpose of raiding the Central Plain was not simply for goods, but to force the Central Plain polity to pay regular tribute. The power of the Xiongnu ruler was based on his control of Han tribute which he used to reward his supporters. The Han and Xiongnu empires rose at the same time because the Xiongnu state depended on Han tribute. A major Xiongnu weakness was the custom of lateral succession. If a dead ruler's son was not old enough to take command, power passed to the late ruler's brother. This worked in the first generation but could lead to civil war in the second generation. The first time this happened, in 60 BC, the weaker party adopted what Barfield calls the 'inner frontier strategy.' They moved south and submitted to the dominant Central Plain regime and then used the resources obtained from their overlord to defeat the Northern Xiongnu and re-establish the empire. The second time this happened, around 47 AD, the strategy failed. The southern ruler was unable to defeat the northern ruler and the Xiongnu remained divided.[149]