After the battle of Hexi and the surrender of the evil king of Xiongnu, the Western Han Dynasty settled them in five counties: Longxi, Beidi, Shang Jun (now Yulin, Shaanxi), Shuofang and Yunzhong, and settled in the area south of the Yellow River beyond the Great Wall, which was known as the "five vassal countries" in history. After this campaign and six years (BC 1 1 1), Han gradually established four counties in Hexi area, namely Wuwei (now Minqin area in Gansu province), Zhangye (now north of Zhangye in Gansu province), Jiuquan (now Jiuquan in Gansu province) and Dunhuang (now Dunhuang west in Gansu province), which were called "Hexi four counties" in history. In this way, Hexi area, which has long been regarded by Huns as the source of war to attack the Central Plains from the west, has gradually become a stable agricultural production area. Later, the Western Han Dynasty moved more than 720,000 poor people from Kanto and settled in Longxi, Xihe, Shang Jun and Beidi. These measures are undoubtedly of great strategic significance for consolidating Hexi and Henan regions and developing local social production. Because the Han Dynasty won the Hexi area and basically wiped out the Xiongnu forces in the west, the soldiers in Longxi, Beidi and Shang Jun counties were reduced by half.

The Xiongnu slave owners' ruling group was helpless to the fiasco of its right part and to the situation that the Han Dynasty was facing an increasing blow. At this time, Weng Hou Zhao Xin, who surrendered to the Huns in the war south of the Han-Hungarian desert, had married Khan's sister, was highly valued by Khan, and was regarded by the Huns as a prominent figure second only to Khan. He suggested to Khan that our army retreat to the north of the desert and lure the Han army to attack. When the Han army is exhausted, another attack will inevitably kill the Han army. This suggestion was highly valued by Khan.

Xie Shan, a Xiongnu, was very angry at the annihilation of the right member. In the autumn of the third year of Emperor founding ceremony (BC120th), two armies attacked Beiping and Dingxiang counties, each with tens of thousands of riders, killing more than 0/000 Han officials.

After the elimination of the right forces of the Huns in the Han Dynasty, the threat from the West was reduced. The Xiongnu Zuo and Khan in the northeast still have considerable strength, and they are still powerful forces that threaten the security of the northern border of the Han Dynasty and prevent the Western Han Dynasty from unifying the northern part of China. In order to promote the unification of northern China and completely eliminate the slave-owner regime of Xiongnu nobles, after nearly two years of economic development, tax reform, wealth accumulation, and careful preparation, the Western Han Dynasty decided to crack down on the forces of Xiongnu Khan and the wise kings around Northeast China.

Occurrence time: four years of yuanshou (BC 1 19)

In the sixth year of Yuan Shuo (BC 123), after the Monan Conference, one of the purposes of the Xiongnu branch Khan's withdrawal from Mobei was to try to "lure the Han soldiers out and take them". Unexpectedly, Emperor Wu of the Han Dynasty turned to Hexi and defeated the Huns' plan to lure troops. Angry and angry, Xie Dan sent tens of thousands of cavalry from Youbeiping and Dingxiang counties in the spring of the third year of Yuanshou (before 120), killing more than a thousand people in an attempt to anger Emperor Wu of the Han Dynasty, induce the Han army to advance northward and annihilate in Mobei.

In view of the fact that Xiongnu Khan still has considerable strength in the headquarters and the left, which seriously threatens the security of the northern border of Han Dynasty, and considering that the Han army has accumulated operational experience in using large-scale cavalry groups to attack from a long distance in many past military exercises, Emperor Wu of Han Dynasty decided to take advantage of Hexi's new victory sail to strengthen the attack on the northern line. In the fourth year of Yuanshou (before 1 19), he issued imperial edicts, carried out monetary reform, ordered "initial calculation of money" and monopoly of salt and iron, and raised a lot of material and financial resources needed for the war. At the same time, discuss with the generals the operational guidelines for the Huns. He believes: "Zhao Xin is a Khan's plan. It is often thought that Han soldiers can't stay in the desert, but they will get what they want when they die today." Determined to take advantage of Zhao Xin's misjudgment, take him by surprise and attack him unprepared, so as to determine an operational policy of concentrating forces, going deep into Mobei and searching for the main force of Xiongnu. Emperor Wu of the Han Dynasty mobilized cavalry140,000, war horses140,000, infantry and transport soldiers100,000. Under the command of Wei Qing and Huo Qubing, it was the largest expedition in Mobei.