Legalism began during the Warring States era of China, between 475 and 221 B.C., according to Encyclopedia Britannica. Because of the conflicting, chaotic nature of this period, the utilitarian precepts of legalism caught on with the ruling class of the Qin Dynasty.
Some scholars note the existence of legalist ideas as early as the writings of Kuan Chung in the 7th century B.C.; however, legalism in its systematized form is a product of the Warring States era, a time of interfaction conflict that resulted in the establishment of the Qin Dynasty. Shang Yang was one of the most important advocates of the philosophy. He not only expounded the principles of legalism in works like "The Book of Lord Shang" and "Shen Pu-hai" but also put them into practice as governor of the state of Ch'in.