The Warring States period is conventionally dated from 475 BCE, when the nominal authority of the Zhou royal house had already collapsed into the rule of a shrinking number of increasingly powerful regional states, to 221 BCE, when the state of Qin completed its conquest of its six remaining rivals and its ruler took the title First Emperor. Seven major states, Qin, Chu, Qi, Yan, Han, Zhao and Wei, fought an escalating series of wars using large conscript infantry armies, iron weapons and new administrative techniques that replaced the older aristocratic chariot warfare of the earlier Spring and Autumn period, while the Zhou kings themselves retained only ritual prestige and no effective power.
Qin's eventual victory owed much to the Legalist reforms of the statesman Shang Yang, adopted in the fourth century BCE, which replaced hereditary privilege with a system of military rank earned through service and harsh, uniformly applied law, giving Qin an administrative and military efficiency its rivals could not match. The period's warfare and political theorizing about how to govern effectively and hold a state together under pressure are inseparable from its philosophical output, since most of the era's major thinkers were also, in one way or another, advisers or critics addressing rulers on exactly that question. The Cambridge History of Ancient China, edited by Michael Loewe and Edward Shaughnessy, remains the standard scholarly reference for the period's political history.