The Battle of Altai Mountains (Chinese: ), was a major expedition launched against the Northern Xiongnu by the Han Dynasty in June, 89 AD. The battle was a success for the Han under Dou Xian (d. 92 AD).
In June 89 AD, the Han dispatched a force which promptly advanced from Jilu, Manyi and Guyang in three great columns that included their allies, specifically the main army of the Southern Xiongnu. The force of General Dou Xian advanced towards the Northern Chanyu into the Altai Mountains. A large detachment then moved to the northwest, and in the major battle of the campaign they defeated the Northern Chanyu at the Altai Mountains and pursued him westwards. The Han forces killed 13,000 Xiongnu troops and accepted the surrender of 200,000 Xiongnu from 81 tribes.
Dou Xian brought the main body of his troops in triumphal progress north to the Khangai Mountains, west of present-day Kharkhorin. There he carved the cliff Inscriptions of Yanran, composed by his client, the historian Ban Gu, which celebrated the achievement of the battle. This inscription was identified in Dundgovi Province by scholars from Mongolia and China in August 2017.
After the successful campaign of 89 AD, the Xiongnu state was destroyed. After the battle, Dou Xian led his forces back, and the Northern Chanyu sought to negotiate peace. Tuntuhe Chanyu of the Southern Xiongnu, however, was anxious to destroy his rival completely, and early in 90 AD, as embassies were still being exchanged, Dou Xian launched an attack, captured his rival's seal and treasure and his wives and daughters.
General Dou Xian soon initiated a punitive expedition against the remaining hostile Xiongnu tribes, subsequently causing the tribes to flee westward. Dou Xian had reported that the Northern Chanyu was so weak there was no point in treating with him further. In February 91 AD, he mounted a final invasion, with two of his generals Geng Kui and Ren Shang in charge. They advanced from Juyan and defeated the chanyu, captured his mother and killed 5,000 of his armies, drove him in flight again to the west from Altayn Nuruu. He was not heard of again. The rest of the Xiongnu left in Dzungaria, specifically near Lake Barkol, had not been directly affected, and some part of the shattered polity was reconstructed under a new chanyu. The new chanyu, however, was killed in 93 AD, and after him no chanyu of the Northern Xiongnu was ever heard of again. On the frontier of China which faced present day Mongolia, the hostile Xiongnu state was ended.