Having ended the civil war, Emperor Gaozu attempted to fortify the northern defenses previously built by the famous Qin General Meng Tian. On the other hand, the Xiongnu, taking advantage of the civil war, had captured bases in the south of the difficult Gobi Desert, and they did not want to lose this place.

Modu Chanyu formed an army from the nomadic coalition he had just formed and crossed the Chinese border. The operation looked like a simple plunder, but Modu Chanyu, a true tactician, had his main goal to intimidate his own people, the nomads he subjugated and the Chinese. With the booty to be obtained, he would both ensure the loyalty of the other tribesmen in the coalition, frighten them with his power, and show a golden stick to the Chinese who started to mobilize in the north (in the south according to the Xiongnu).

Under his adventurous appearance, Modu Chanyu was a cautious strategist, being careful not to get trapped inside China. He besieged Mai Fortress, the most important point in China's northern defense. The defender of the castle was Xin of Han, a relative of the emperor (not to be confused with Han Xin). When the prince lost hope of help, he surrendered and entered the service of Modu Chanyu.

Modu Chanyu had achieved his goal and succeeded in provoking Emperor Gaozu. The Emperor set up a huge army and moved north to fend off the Xiongnu threat.

Modu Chanyu pulled his army into the mountains, while attacking left and right, plundering everything that belonged to the Chinese. He made the Emperor follow him by creating the impression of a ragged raider, in fact what he had been doing all along was luring the Chinese army into a trap.

The entire Xiongnu army consisted of cavalry and they were prepared for the cold as they came from the continental climate in the north. He had gloves and clothes suitable for winter conditions. The Chinese soldiers, on the other hand, were infantry recruited from the common people and fell into the grip of the cold while chasing the Xiongnu across the high mountains. The fingers of thousands of Chinese archers froze, and a significant portion of China's military force was rendered incapable of combat.

This was the first piece of Modu Chanyu's trap.

Modu Chanyu quickly pulled back his cavalry, giving the impression that he was "escaping from the hands of the Chinese". The emperor was alarmed, for he had two options before him. Either he would let the Xiongnu go, or he would simply take the fast troops with him and go after them. Emperor Gaozu chose the second path, which was risky.

The Chinese army was split in two, and the Emperor pursued the Xiongnu at the head of a small but fast unit.

Modu Chanyu, who was retreating, suddenly stopped and attacked the Chinese. The Chinese were stunned by this unexpected move, they encountered the Xiongnu in front of Baideng Fortress and suffered a heavy defeat.

The Battle of Baideng is the first example of the "feigned retreat" tactic used by nomads throughout history.^{[citation needed]}

The emperor retreated to the castle in panic. He hoped to rally his troops and assess the situation, but the Xiongnu seized the opportunity and surrounded the fort. Mete divided his army into four divisions and kept all the exits of the castle.

The presence of the armies is uncertain. The total strength of the Chinese army was over 300,000, but it is not known how much of the vanguard was trapped in the fort with the Emperor. Although Chinese historians write that the number of the Xiongnu army was 400,000,^[4] this number is greatly exaggerated, since the total population of the nomads would not be enough to raise such an army, and since each soldier had at least two horses and these horses would be scattered around the castle and fed with grass during the siege, the size of the Xiongnu army should have been roughly limited by the amount of meadows. Accordingly, the number of the Xiongnu army must be between 20,000 and 40,000.^{[citation needed]}

The siege lasted seven days. The emperor was cornered and was willing to accept all conditions for peace. Even if he got out of here, he knew that he would not be able to continue the war because he had witnessed the tactics and maneuverability of his enemy. The emperor offered peace to the Xiongnu.

According to Chinese historians, the yabghu's wife, "aunt", was contacted and deceived with gifts. And under the influence of his wife, the yabgu accepted the peace. However, if we consider Modu Chanyu's genius, there may be another explanation for the situation. Modu Chanyu had struck fear into the hearts of the Chinese, but he was aware that his own strength was limited. He would not be able to maintain the siege for long, and if he encountered the main part of the Chinese army coming from behind, he could be caught between two fires and trapped himself. Also, the peace offer was quite reasonable, and Mete was too realistic a leader to attempt to invade China with a handful of cavalry. As a result, he accepted the Chinese's terms, perhaps creating the impression that he had agreed to peace at his wife's plea, so as not to undermine the show of force.^{[citation needed]}

Modu Chanyu applied the psychological attrition tactic to the end. After the treaty, he allowed the Emperor to leave the castle, but he placed his soldiers on both sides of the Chinese road and had the arrows pointed at the Emperor. Gaozu would not forget this fear for the rest of his life and would surrender to all of Mete's threats.^{[citation needed]}

Modu Chanyu did not neglect to renew the fear he left in the heart of the enemy by organizing arbitrary raids on China afterwards. However, the Chinese kept the treaty as long as Mete was alive. According to this, they gave annual tax to the Xiongnu under the name of "gift", left the trade and defense region in the north to the Xiongnu and gave a princess to the yabghu.

• Han-Xiongnu wars
• Xiongnu invasion of Donghu

• Yap, Joseph P. pp 71–76. "Wars With The Xiongnu, A Translation From Zizhi tongjian" AuthorHouse (2009) ISBN 978-1-4490-0604-4