Modu Chanyu
The Qin general Meng Tian had forced Toumen, the "Chanyu" of the Xiongnu, out of the Ordos Desert in 215 BCE, but Toumen's son and successor Modu Chanyu built the Xiongnu into a powerful empire by subjugating many other tribes. By the time of Modu's death in 174 BCE, the Xiongnu domains stretched from what is now northeast China and Mongolia to the Altai and Tian Shan mountain ranges in Central Asia. The Chinese feared incursions by the Xiongnu under the guise of trade and were concerned that Han-manufactured iron weapons would fall into Xiongnu hands. Gaozu thus enacted a trade embargo against the Xiongnu. To compensate the Chinese border merchants of the northern kingdoms of Dai and Yan for lost trade, he made them government officials with handsome salaries. Outraged by this embargo, Modu Chanyu planned to attack Han. When the Xiongnu invaded Taiyuan in 200 BCE and were aided by the defector King Xin of Hán (韓/韩, not to be confused with the ruling Hàn 漢 dynasty, or the general Han Xin), Gaozu personally led his forces through the snow to Pingcheng (near modern Datong, Shanxi). In the ensuing Battle of Baideng, Gaozu's forces were heavily surrounded for seven days; running short of supplies, he was forced to flee.