The Warring States period saw the fruit of the technological and economic advancement under the comparative peace of the Zhou dynasty. Iron began to be used in implements, and animal drawn plough, crop rotation, extensive irrigation works allowed agricultural production to boom, and thus population. The crossbow replaced the compound bow (Turkish bow) as the weapon of choice.
Another key military advancement was the introduction of cavalry, borrowed from the nomadic horsemen of the north. Initially derided by Chinese lords as barbaric, horsemen allowed the advancement of war to greater heights, and brought low charioteers, who were often aristocrats.
On a sidenote, I have read in a few places that many of the ingredients for an industrial revolution were present from around 200 BCE onwards. Metal working technology had reached a height that western civilisation would not reach for almost a millenium. There was a large and rapidly urbanising population. There was increasing technological sophistication. There was money, writing, booming population. It boggles the mind that all this happened several centuries before the birth of Christ, when most western civilisation were just establishing their roots. It seems that some vital ingredient was missing.
The royal domain was gradually usurped, and disappeared in about 250BCE, and the Zhou line died out in 249BCE, ignored, and of no relevance to the growing power of the states.
The wars are numerous, and many atrocities were committed during this time. Chinese history is extraordinary bloody at this point. The state of Qin, through a series of military successes, conquered the rich state of Chu, in the Yangzi river basin, and picked off the other states to the north one by one, until the last, Chi, surrendured to their forces without a fight. From all accounts, the ruler of Qin, Qin shihuangdi systematically executed and "extinguished" all the royal lines.
The fall of Chin marks the end of the Warring states period, and the establishment of the first "imperial" dynasty, Qin.