Beginning in 324 BCE three northern Chinese states with nomadic neighbors—Qin (Ch'in), Zhao (Chao), and Yan (Yen)—began to build defensive walls. After Qin unified China in 221 BCE the first emperor ordered his most able general, Meng Tian (Meng T'ien), to connect these existing walls and extend them to form a unified system of defense. The result is the Great Wall of China.

Upshur, Jiu-Hwa Lo. "Great Wall of China." Encyclopedia of the Ancient World: Prehistory to 600 CE. Facts On File, 2016. Ancient and Medieval History Online. Retrieved from http://db.corpus.wa.edu.au/login?url=http://online.infobase.com.db.corpus.wa.edu.au/Auth/Index?aid=102119&itemid=WE49&articleId=237699