Warmatrix

China was first unified under the Qin, a people from the western part of the area. The process took almost three centuries, accelerating in the last one.
In ancient times China was divided into many states, some large and some small. In the early Bronze Age the Shang dynasty rose to a position of dominance, strong enough to be listed as the first dynasty of China by historians. They controlled a large area, though nowhere near to the size of modern China. The Shang were replaced by the Zhou, who were the first to claim the 'mandate of heaven' that would become the cornerstone of many a future Chinese government. The Zhou later split into a western and an eastern half. Later still, their empire disintegrated during the Spring and Autumn period, which was followed by the Warring States period. As the name implies, in this period many states fought each other: Yan, Jin, Qi, Chu, Qin and many smaller ones. Eventually the Qin managed to gain the upper hand and conquer all others.
The Qin were established in 771 BCE by duke Xiang, who supported king Ping of the Zhou and was rewarded by the status of a vassal state. The western part of China, where Qin was based, was less fertile than the east. The population consisted mostly of semi-nomadic people, not very sophisticated. During the early Warring States period Qin suffered some losses at the hands of their enemies. When duke Xiao came to power in 361 BCE, he abolished most prerogatives of the nobility, granted freedom to slaves, set up a judicial system and started promoting military officers based on performance, not hereditary ties.
The reforms had a profound effect. The industrious population constructed many public works that provided support for the army, which became very competent. In 318 BCE, an alliance of Wei, Zhao, Han and Chu attacked Qin, but failed to achieve anything. Qin went to the offensive and managed to take much territory from the powerful Chu state in the south. Next, states on the northeast border were attacked and reduced to buffers, while the Qin maintained good relations with other states near the coast. From 262 BCE to 260 BCE Qin fought Zhao and overcame them, but only after a mobilization on a massive scale, like the total wars of the 20th century CE. It reinforced its power base by digging canals and setting up irrigation systems that produced food to sustain large armies. From 237 BCE to 221 BCE, the latest Qin king Ying Zheng launched a 16-year long war to unify China. Qin attacked and conquered its rivals one by one, despite suffering a single large defeat by the hands of the Chu.
The Qin, like their adversaries, used massive armies of sometimes 200,000 men, supported by the fertile soil of China's agricultural lands. Infantry was the mainstay, combined with cavalry and there were still some chariots lingering around. In sieges they used massive fortress ships and to counter these, fireships.
The success of the Qin conquest was such that Ying Zheng could call himself the first emperor of China, (Qin) Shi Huangdi. It said that he spent the last years of his life searching for an elixir of immortality. Failing that, he immortalized himself in a different way, by filling his tomb with a vast terracotta army of lifelike statues of soldiers.