Various Ordos bird ornaments from the 5th C. BCE - 3rd C. CE

In the year 9 CE the Wang family, led by Wang Mang, overthrew the Han Dynasty and established the Xin Dynasty.

Wang Mang quickly led China on a collision course with its subjects and allies, particularly the Huns. One of Wang Mang's officers notified the Wuhuan that they didn't need to pay tribute to the Xiongnu anymore, which they had been doing for the past 200 years. This led to a conflict between the Xiongnu and Wuhuan. In the year 9 CE he ordered the Xiongnu Chanyu's imperial seal confiscated and replaced by a commoner stamp with a degraded title: "Wang Mang decided to change the title of Xiongnu Chanyu to Xiangnu fuyu the Surrendered Slave that Submitted" (from the Hanshu). 

The Xiongnu were already feeling disrespected prior to these changes. In 8 BCE the Han government had attempted to take Xiongnu land near the Zhangye Prefecture. In 1 CE the Indo-European Jushi people (under King Guju) grew tired of their Chinese overlords and sought refuge with the Xiongnu. In the same year, King Quhulai of the Ruo-Qiang (probably proto-Tibetans) and his people sought refuge with the Xiongnu because the Chinese officials refused to protect them from the Chi-Qiang's attacks. Wang Mang was already in an executive position in the Han Court by this point when they gave orders to the Xiongnu to turn over their refugees, reminding the Chanyu that they could exterminate the Xiongnu: "The Chanyu was much alarmed by the reproach, he went down on his knees, kowtowed supplicating for pardon...he dispatched envoys and pleaded on behalf of the kings". The Han Court, heavily under Wang Mang's influence ignored the pleas and had both kings executed in front of a gathering of all the kings in the Western Regions. There was previously no agreement that the Xiongnu couldn't provide sanctuary to the Jushi or Qiang peoples so the Han Court passed a new law forbidding it. 

The Chanyu, Luanti Nangzhiyasi, then began organizing his military and sent 10,000 soldiers to Shuofang to camp outside China's walls to show his frustration. Wang Mang had been on the throne for less than a year and the Xiongnu had already had enough. 

-Wars with the Xiongnu, a translation from Zizhi Tongjian, by Joseph P. Yap

-The Western Regions, Xiongnu, and Han: from the Shiji, Hanshu, and Hou Hanshu, translated by Joseph P. Yap

Source:

https://www.sothebys.com/en/buy/auction/2022/chinese-art-online-a-private-asian-collection/a-group-of-10-ordos-bronze-bird-ornaments-warring

Quote:

Wars with the Xiongnu, a translation from Zizhi Tongjian, by Joseph P. Yap

The Western Regions, Xiongnu, and Han: from the Shiji, Hanshu, and Hou Hanshu, translated by Joseph P. Yap