This period in Chinese history has a reputation as a turbulent one, as its name, the “Warring States”, suggests (sources disagree on the exact dates, but most state the start of this period as 481/479 BC and the end is almost universally said to be 221 BC). The death of King Zhou (who ruled over the previous “Spring and Autumn” period) signalled the end of a time of relative stability in ancient China and thus chaos ensued, although the Zhou dynasty is mostly not recognised as ending until the end of the Warring States. Seven main states known as the “Seven Overlords” emerged, those of Ch’u, Q’in (or Ch’in), Wei, Han, Ch’i, Yen, and Chao. Society had changed substantially, as these states replaced the warlords who dominated in the Spring and Autumn, and the nature of warfare had also changed drastically by the time of the Warring States.
War in the Spring and Autumn period had been almost ritualistic, and followed a strict code along the lines of the European chivalric ideas that the west is more familiar with. This involved seasonal campaigns and "rules" such as not striking elderly men in battle or those already wounded and on the ground, and it was seen as morally wrong to “massacre cities” or “ambush armies”. Feudal armies were the only kind, in that men had to fight for their lord when called upon and no professional formations existed, with battles being short, indecisive melees. Victory was rarely exploited, as objectives were limited to gains of land in the minds of the Dukes who led their armies.
By 500 BC, however, warfare had become much more ferocious – there is an example of three thousand condemned men lining up in front of the enemy and, when ordered, committing ritual suicide by slitting their own throats on the spot. The enemy, not suprisingly, is said to have fled. Society had changed so that well-trained, disciplined armies became the norm, and the days of chivalry and the knight were over. This possibly reflected the greater frequency of conflict and is reflected by the construction of walls around several states for protection; the beginnings of the Great Wall. The writing of several tomes on war (Sun Tzu's The Art of War being the most famous) also shows that warfare was a major part of Chinese life at this time.
The pacifist, anti-war writings of Mo Ti also appeared at this time, and showed deep understanding of both the nature of war and the impact on the civilian population (90% were peasants, acting as serfs under the feudal system) conflict would have. The morals and ethics of war were also discussed by Mo Ti, his view being that all men were equal under Heaven, and activities such as offensive war went against his "universal love" philosophy as it involved hurting other people.
Despite the endemics wars raging in this period, many technological advancements were made, and significant changes took place in the field of philosophy. Metal working became easier and therefore more common due to a new type of forge, and agriculture was aided by inventions such as a new form of plow. Schools of thought such as Confucianism continued to grow despite rivals, an example being Mohism founded by Mo Ti.
So, the Warring states period was one of transition, from loose collection of small state to Empire. The period was born out of war and died through war, as the First Emperor-to-be conquered the other states by 221 BC to form a more unified China, albeit still much smaller than the one which exists today (the states were concentrated in the east, near the coast). This then heralded the start of the Qin or Ch'in period of Chinese history.
Thanks to Samuel Griffith's 1960s translation of The Art of War and the accompanying notes for much of the information in this writeup.