The Warring States period is the closing era of the Eastern Zhou dynasty in China, conventionally dated from about 475 BCE to 221 BCE. Seven large kingdoms, Qin, Chu, Qi, Yan, Zhao, Wei and Han, absorbed their smaller neighbours and fought each other with mass conscript armies while the Zhou king kept his sacred titles and almost no power. It was also the age of the Hundred Schools of Thought, and it ended when Qin conquered the last of its rivals and founded China's first empire.
A nobleman needed years to learn to shoot accurately from a chariot at the gallop. A conscript could be handed a crossbow and made useful with far less training, because a bronze catch held the bowstring at full draw and asked mainly that he aim and release. That mechanism was one piece of a larger change. Iron was cheapening blades and farm tools, states were registering households and taxing harvests directly, and armies were swelling from noble retinues into massed infantry stiffened by cavalry and sustained by long supply lines. Together these turned war from an aristocratic contest into an accounting problem in men, grain and iron. The states that solved the accounting swallowed the states that did not.