When the bigger states threw off Zhou rule in the 8th century BC they fought amongst themselves for dominance. Over the next five centuries wars became longer and involved more and more troops.
New weapons appeared: the crossbow and catapult, walled fortifications, methods of city siege and defence and, gradually, iron weapons. Useful ideas were copied from Mongolian horsemen, like the saddle, and trousers instead of robes, to give greater agility on horseback and in battle. To speed troop movements roads were built.
These new methods of war in the Eastern Zhou state between the 8th and 3rd centuries BC made it the most dynamic military power in the world. War was no longer a chivalrous ritual for noblemen, but a serious, bloody business fought by professional generals commanding huge armies.
Gradually the state of Qin achieved dominance, mainly because of their introduction of the longer iron sword and the terrorizing ferocity of their army. After one battle the Qin cut off the heads of 400,000 Chao prisoners.
