Qin's campaign against the Xiongnu


In 215 BC, Qin Shi Huangdi ordered General Meng Tian to set out against the Xiongnu tribes in the Ordos region, and establish a frontier region at the loop of the Yellow River. Believing that the Xiongnu were a possible threat, the emperor launched a preemptive strike against the Xiongnu with the intention to expand his empire.
Later in the year, Meng Tian succeeded in defeating the Xiongnu, driving them from the Ordos and seizing their homeland. After the catastrophic defeat at the hands of Meng Tian, the Xiongnu leader Touman was forced to flee far north into the Mongolian Plateau. As a result of the northward expansion, the threat that the Qin empire posed to the Xiongnu ultimately led to the reorganisation of the many different Xiongnu tribes united into a confederacy against the unified Chinese state.

Aftermath

Following the victory against the nomads, Meng Tian was instructed to secure the frontier with a line of fortifications, which would become known as the Great Wall of China. Crown Prince Fusu and General Meng Tian were stationed at a garrison in Suide and soon began with the construction of the walled defenses, which would be connected with the old walls from the Qin, Yan, and Zhao states. The Qin walls ran from Liaodong to Lintao, thus enclosing the conquered Ordos region. However, the garrison was abandoned from the civil war after the death of Qin Shi Huang leading the dynasty's collapse. The chaos of the interregnum Chu–Han Contention allowed the Xiongnu under Modu Chanyu to recapture the territory that they had previously lost to the Chinese and to conquer and subjugated other nomadic tribe such as the Yuezhi and Donghu. Before long, they began raids into China under the newly established Han dynasty leading up the Emperor Gaozu of Han to launch a campaign against them at the Battle of Baideng, which result into total humiliating defeat by the Nomads.