Xiongnu plaque of four tigers attacking an ox 3rd C. BCE

The year 71 BCE began one of the worst moments in the history of the Huns. In this year the Wusun and Han attacked the Xiongnu, delivering a devastating defeat. From the Book of Han: "the number of Xiongnu people killed, wounded, and dispersed could not be estimated. The loss of their livestock through these encounters was inestimable. Xiongnu began to decline, and the state became emaciated, and they blamed it on the Wusun."

That winter Huyandi Chanyu retaliated against the Wusun and was victorious but a snowstorm occurred during the expedition that resulted in over eight feet of snow in a single day and less than ten percent of the tens of thousands of deployed Xiongnu soldiers survived the storm. 

Shortly after the survivors made it home they were attacked from all sides by their enemies; from the north by the Dingling, from the east by the grave-desecrating Wuhuan, from the west by the duplicitous Wusun, and from the south by the Han Empire. The Book of Han states tens of thousands of Xiongnu were killed in this war: "Resulting from these clashes three out of ten Xiongnu people died, and half of their livestock also died." The numerical losses from this event and those witnessed from the wars immediately before/after seem to indicate that it wasn't just soldiers that were targeted but all Xiongnu civilians, including women and children. 

In 68 BCE the Xiongnu were hit by a severe drought that killed most of the survivors of the previous war: "...six or seven out of ten people died, and animal losses were similar. The Chanyu then issued several tens of thousands of cavalrymen from two garrisons to be in battle readiness waiting for the Han army to invade...". 

A few years after this event the Xiongnu government began to unravel. As bad as this time was, the Xiongnu would make a remarkable recovery. Just a few more years after this, the Chinese turned on the Wusun, splitting their nation between two opposing factions and keeping them in a state of constant civil strife. Just as the Wusun had betrayed the Huns when they no longer needed them, the Chinese betrayed the Wusun. The exact reason for this isn't clear, some writings make it sound like the Chinese were legitimately trying to help mediate the situation, though their actions didn't look that way at all. The Chinese probably thought the Huns were defeated to the point that they could keep them pacified on their own. At the same time the Chinese probably didn't trust the Wusun. 

Han Governor of the Western Regions, Guo Shun, wrote that even after the heqin marriage arrangement the Wusun were a constant problem for China and looking for any excuse to start a war. The Huns had given the Wusun national independence from the Yuezhi invaders and superior Hun military tech/training, yet the Wusun still betrayed them for Chinese gold. The only thing more the Chinese could offer the Wusun was money and economic benefits, but it would be undesirable to have a wealthy and powerful nation on your border that has a history of attacking its benefactors. It was probably only a matter of time before the Wusun turned their erratic behavior against the Chinese, hence why the Chinese politically positioned the Wusun state to encourage infighting. Any delusions of grandeur the Wusun had seems to have began its steep decline here.

A couple years after this the Huns defeated the Dingling under Zhizhi Chanyu. A few decades later the Han Dynasty would collapse and the Wuhuan willingly offered themselves, in shame, as vassals to the Huns. 

-The Hanshu (Book of Han) Xiyu Zhuan Volume 94 Number 64 Part A. Translation and notes by Joseph P. Yap: The Western Regions, Xiongnu, and Han. 

Xiongnu plaque of four tigers attacking an ox, 3rd century BCE. Gold, 12.7 x 7.4 cm. Excavated at Aluchaideng, Hangjin Banner, Ih Ju League (Inner Mongolia). National Museum of China, Beijing.

Source:

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Quote:

The Hanshu (Book of Han) Xiyu Zhuan Volume 94 Number 64 Part A. Translation and notes by Joseph P. Yap: The Western Regions, Xiongnu, and Han.