1.  Introduction

History did not have the haphazard, all things that happen in history must have the portent. History has long foreshadowed the decline of the Xiongnu Empire, which once engulfed Eurasia. When we review the history of the rise and fall of this huge nomadic dynasty, we clearly see that the military attacks of the Han Empire, as well as internal divisions among the Xiongnu society, together constituted the toxin that constantly eroded the once "giant"—Xiongnu Empire. Its impact is far-reaching and has reshaped the political landscape of East Asia and Inner Asia. This paper will give a systematic explanation and support that why the military attacks of the Han Empire, as well as internal divisions among the Xiongnu society are the most important reasons of the decline of Xiongnu Empire. This paper systematic analysis why the internal division of the Xiongnu and the strikes by Han Dynasty is the most important reason of decline of the Xiongnu.

2.  Counterattack and the attack of the Han Empire

The Han Empire and the Xiongnu Empire were the hegemon of Asia at that time(202 BC to 220 AD). However, in the early days of the Han Empire, due to the internal instability brought about by the Chu-Han War that had just ended, most of the wars between the Han Empire and the Xiongnu Empire ended in failure [1]. The Siege of Baideng clearly illustrates the Han Empire's inability to cope with the Xiongnu in its early years. Therefore, at the beginning, the Han Empire adopted strategies such as defense and marriage alliances [2], as well as the Taoist philosophy of "following nature and governing by inaction" to quickly accumulate strength [3] while avoiding conflict with the Xiongnu Empire. Although the Han Dynasty's early marriage alliance policy and defensive strategy provided a respite, the Xiongnu continued to frequently raid southward (for example, during the reign of Emperor Wen, the Xiongnu invaded Longxi and Yunzhong). In the 14th year of Emperor Wen's reign (166 BC), 140,000 Xiongnu cavalry even attacked Ganquan Palace near Changan [2]. Therefore, during the reign of Emperor Wu of Han, Liu Che, relying on the huge wealth accumulated during the reigns of Emperor Wen and Emperor Jing [3] and a large number of military talents (such as Wei Qing and Huo Qubing) [2], in order to seize the Hexi Corridor and the passage to the Western Regions and to resolve the huge threat posed by the Xiongnu to the Han Dynasty [4], the Han Dynasty decided to switch its defensive policy towards the Xiongnu to an attack strategy [5]. Obviously, the Han Dynasty's attack was effective. 119 BC, during the War of Mobei, Wei Qing and Huo Qubing led their armies deep into the desert, defeating the main Xiongnu force and forcing the Chanyu to flee [2]. This War severed the Xiongnu's connection with the Western Regions, captured the Hexi Corridor, and established the Han Dynasty's strategic advantage over the Xiongnu [5]. In 119 BC, Emperor Wu of Han dispatched Huo Qubing to lead his army deep into the northern desert. At Langjuxu Mountain, he held a ceremony to offer sacrifices to heaven. This war completely routed the Xiongnu army led by Zuo Xianwang, annihilating over 70,000 of them and capturing many Xiongnu nobles [2]. [3] Wei Qing's army beheaded 19,000 people and burned four granaries at Zhaoxin City. Huo Qubing's army beheaded 70,443 people and sealed Wolf Lair Mountain 14 times. Wei Qing's army lost about 9,000 men. Huo Qubing's army lost about 10,000 men. The Han army lost about 20,000 men, and the Xiongnu lost about 90,000 men [3]. From 121 to 111 BC, Huo Qubing led two expeditions to the west, conquered the Hexi region, established the Wuwei, Zhangye, Jiuquan, and Dunhuang prefectures, and settled immigrants there to cultivate land [2]. In the first battle, 8960 Xiongnu were beheaded and 9 golden statues used by King Xiutu for offering sacrifices to the heaven were captured. In the second battle, 32,000 Xiongnu were beheaded and the Xiongnu kings [2]. Huo Qubing annihilated the forces of King Xiutu and King Hunxie, seized the Hexi Corridor, established four prefectures including Wuwei and Zhangye, and severed the alliance between the Xiongnu and Qiang people, depriving them of their western supply route [3]. In 36 BC, Chen Tang, a general of the Western Han Dynasty, led his army on an expedition to Kangju (present-day Kazakhstan), killed Zhizhi Chanyu, [2] and declared that “whoever openly offends the mighty Han will be punished, even though he is far away.” This war almost wiped out the Xiongnu [3]. Therefore, Han Dynasty's attack greatly weakened the Xiongnu. It reduced the Xiongnu's range of activities and the number of its troops, and after the opening of the Silk Road to the Western Regions, it greatly strengthened the relationship Between the Han empire and the Xiongnu's allies. This shows that the attack by the Han Empire was indeed one of the important reasons for the decline of the Xiongnu.

3.  Internal divisions among the Xiongnu society

If the Xiongnu Empire had been united, the Han Dynasty's attack would not have been so successful, and Xiongnu would not have declined so quickly simply because of the Han Dynasty's attack. Therefore, the internal divisions within Xiongnu must have been an important reason for Xiongnu's decline. A very important reason for the internal divisions within Xiongnu was the flaws in the Xiongnu Chanyu inheritance rules. Xiongnu's inheritance system was "brother to brother succession", which was extremely vague and prone to ambiguity. This led to many conflicts over Chanyu's position. In addition, Xiongnu's own tribal alliance social structure brought more instability. There are many reasons for Xiongnu's internal division. In addition to the flaws of its own system, natural disasters and the Han Dynasty's “divide and control” strategy also played a big role. For the natural disasters, in 71 BC, the Chanyu was on his way back from an expedition to Wusun when he encountered a snowstorm, which resulted in the death of "three out of ten people and one out of ten livestock." In 68 BC, famine killed "six out of ten people and seven out of ten livestock," destroying the Xiongnu's economic foundation [3]. The Han Dynasty supported some pro-Han forces. For example, Emperor Xuan of Han accepted Huhanye Chanyu and granted him the "Xiongnu Chanyu Seal" to establish a vassal relationship. Emperor Guangwu of the Eastern Han Dynasty conferred the title of Nan Chanyu Bi on him and opened the Hetao area for him to graze, making it a barrier for the Han Dynasty [3]. In addition, the Han Dynasty also imposed a large-scale economic blockade on the Xiongnu, such as prohibiting the import of ironware and salt into Mobei, which weakened the supply of weapons to the Northern Xiongnu. Thus, under the influence of many factors, the first major split of Xiongnu broke out. In 60-51 BC, after the death of Xulu Quanqu Chanyu, the Right Wise King Tuqitang colluded with Zhuanqu Yanshi to usurp the throne and established himself as Wuyanqudi Chanyu. He killed the old ministers such as Haosu King and Xing Weiyang, which triggered the split of the aristocracy [3]. After this, Wuyanqudi Chanyu forcibly deposed the son of Zuo Aoqian King and installed his own son as king, triggering a rebellion by the nobles of Zuodi. At the same time, Xianxian Shan, the King of Ruju, defected to the Han Dynasty with tens of thousands of cavalry and was granted the title of "Guide Hou" by the Han Dynasty, weakening the Xiongnu's control over the west [3]. Thus, in 58-56 BC, five Chanyu were in appear in same time. They are Huhanye Chanyu who was supported by the nobles of Zuodi and attacked and killed Wuyanquti Chanyu; Tuqi Chanyu, Wuyanquti's younger brother, who occupied Chanyu's court; and Hujie Chanyu, Cheli Chanyu and Wuji Chanyu who split from Tuqi Chanyu's camp and established themselves [3]. The five khans fought against each other, and the chaos led to the Xiongnu “many leaders and generals led their people to surrender to the Han”, and the population loss exceeded 100,000 [3]. In 56 BC, Huhanye defeated Tuqi Chanyu, but the northern part of the border was still controlled by Zhizhi Chanyu. In 51 BC, Huhanye went south to submit to the Han Dynasty and accepted the "Xiongnu Chanyu Seal" bestowed by Emperor Xuan of Han, thus establishing his vassal state status [3]. This great war that lasted for almost 9 years led to a significant decline in the strength of Xiongnu. Coupled with the Southern Xiongnu who defected to the Han Dynasty, the entire Xingnu Empire was severely damaged and began to decline. This not only dealt a heavy blow to the Xiongnu Empire's economy, but also led to the collapse of its political authority because the Chanyu was demoted from a king on equal footing with the Han emperor to a vassal of the Han Dynasty.

After the first split of the Xiongnu, internal peace lasted for approximately 97 years until the second split occurred between 46 and 48 AD. In 46 AD, Huduershidaogaoruodi Chanyu, breaking tradition, passed the throne to his son, Wudadihou, and murdered his younger brother, Yituzhiyashi, who was supposed to succeed him. This sparked discontent among many nobles. Furthermore, from 46 to 48 AD, drought and locusts plagued the land, leaving thousands of miles of land barren, withered vegetation, and most people and livestock perished from famine and disease (The History of the Former Han Dynasty). The population plummeted by 40%, and livestock by 70%. Tribes plundered each other for survival. So, in 48 BC, the grandson of Huhanye Chanyu united eight noble tribes and proclaimed himself Chanyu, leading 40,000 men south to submit to the Han Dynasty. Emperor Guangwu of the Han Dynasty, Liu Xiu, appointed him the Southern Chanyu and opened up the Hetao area for him to graze. Punu Chanyu led the remaining troops to retreat to the northern desert, where they were subjected to an economic blockade (ban on ironware and salt) and a military encirclement (alliance with the Wuhuan and Xianbei) by the Eastern Han Dynasty [3]. Finally, in the 3rd century AD, Cao Cao divided the Southern Xiongnu, who were attached to the Han Dynasty, into five tribes. The nobles changed their Chinese surnames and intermarried with the Han people. During the Southern and Northern Dynasties, they were fully integrated into the agricultural civilization. The Northern Xiongnu were almost destroyed in the Battle of Yanran Mountain in 89 (Dou Xian and the Southern Xiongnu defeated the Northern Chanyu and "beheaded 13,000 people" and engraved the heads on the Yanran Mountain.) and the Battle of Jinwei Mountain in 91. The remaining remnants moved west to Central Asia and some evolved into the European Huns [6]. At this point, the Xiongnu Empire was completely destroyed.

4.  Conclusion

The decline of the Xiongnu Empire was the result of a combination of internal and external factors. On the one hand, the Han Dynasty's sustained and increasingly intensified military campaigns significantly weakened the Xiongnu's military strength and economic foundation. During the reign of Emperor Wu of Han, a series of strategic campaigns in Henan, south of the desert, Hexi, and north of the desert not only captured the Hexi Corridor and severed the Xiongnu's connection with the Western Regions, but also severely depleted their population and livestock resources, depriving them of their strategic initiative. Furthermore, long-standing structural flaws within the Xiongnu—particularly the power struggles caused by the ambiguous succession system for the Chanyu, the loose nature of tribal alliances, and the frequent impacts of natural disasters—made it difficult for the Xiongnu to develop lasting and stable political cohesion.

More crucially, the Han Dynasty cleverly exploited internal divisions, employing a "divide and rule" strategy to support pro-Han forces, encourage the southern Xiongnu to submit, and impose an economic blockade and military encirclement on the northern Xiongnu, accelerating their political disintegration. Two major splits (60–51 BC and 46–48 AD) not only resulted in the coexistence of Chanyu and the dispersal of their peoples but also fundamentally undermined the Xiongnu's foundation as a unified nomadic empire. The synergistic effect of internal and external factors ultimately led to the irreversible decline of the Xiongnu Empire: the Southern Xiongnu gradually integrated into the Han Dynasty's frontier system, while the Northern Xiongnu dispersed westward under the persistent attacks of the Eastern Han Dynasty. This process not only marked the end of the Xiongnu political entity but also profoundly reshaped the geopolitical landscape of East Asia and Inner Asia, highlighting the structural dilemma faced by nomadic empires in the face of highly organized agricultural civilizations and their strategic competition.