[Why] In the early Western Han, the state had emerged from late Qin warfare with the economy in tatters and the populace fragile. Meanwhile, the Xiongnu cavalry empire on the northern steppe surged in strength under Modu Chanyu and raided the frontier repeatedly. [What] Emperor Gaozu personally led the army out to fight and was unfortunately surrounded by four hundred thousand elite Xiongnu cavalry at Mount Baideng for seven days. The Han army finally escaped by lavish bribes to the Yanzhi consort and diplomatic concession. [Who] Emperor Gaozu Liu Bang underestimated Xiongnu strength and faced grave personal danger in the campaign. Modu Chanyu was a ruthless and capable leader who built the northern nomadic empire to overwhelming military scale. [How] The battle forced the Han to abandon armed confrontation in the early dynasty and to adopt the humiliating but necessary heqin (peace-marriage) policy along with frontier trade. It secured the Han several decades of crucial recovery breathing room.
Why
The historic event of Siege of Baideng represents a key developmental peak of the Huaxia dynastic system. The early Han empire's geopolitical compromise under massive steppe pressure. By establishing this moral or administrative benchmark, it continues to shape the structural and philosophical fabric of ancient Chinese statecraft.