[Why] After unification, the powerful Xiongnu cavalry empire raided southward repeatedly into the Guanzhong and Yan-Zhao frontiers as they recovered from war. Qin required a fixed northern defense line. [What] Qin Shi Huang sent General Meng Tian with three hundred thousand troops to drive back the Xiongnu and recover the Ordos Loop. He then mobilized hundreds of thousands of laborers to link and extend the old defensive walls of Qin, Zhao, and Yan into the Great Wall stretching ten thousand li. [Who] Meng Tian was a brilliant general who terrified the Xiongnu, leading the initial wall-defense architecture. Countless nameless conscript commoners poured out their blood and sweat in the harshest natural conditions. [How] The Great Wall successfully blocked nomadic incursions and protected the development of Central Plains agriculture. It became the eternal geographic boundary marker dividing the agricultural and pastoral civilizations of East Asia.
Why
The historic event of Building the Great Wall represents a key developmental peak of the Huaxia dynastic system. Establishing the classic border between agricultural and nomadic civilizations. By establishing this moral or administrative benchmark, it continues to shape the structural and philosophical fabric of ancient Chinese statecraft.