In 221 BC, the first Emperor of China (so-called because all the previous dynastic heads were referred to as kings), Qin Shihuangdi, conquered the rest of China after a few hundred years of disunity. There are two major reasons why he won; the first is that he was a devout Legalist (so much so that he burned all, or at least what he thought were all, the books in the country and more drastic actions like executing generals for reporting late for maneuvers which later proved to be his downfall. The other reason is because the state of Qin had a lot of iron, and consequently, at the dawn of the Iron Age, had an arsonal of more iron weapons than the other armies had. Qin Shihuangdi had many great accomplishments, not the least of which was the linking together of many of the old packed-earth defensive walls from the old principalities into the Great Wall of China. This is not to say that he built the massive masonry construction that today is called the Great Wall of China was actually built close to two thousand years later and the original was during the Ming dynasty.

In the year 210 BC Qin Shihuangdi died. It wasn't long before the dynasty fell apart, helped in part by a revolution started by a soldier who, fearful of execution because he was going to be late delivering a group of new draftees (it had been very rainy and the roads had turned to mud), convinced his conscripts to rebel with him (since they faced the possibility of execution). They were eventually caught and duly executed but their rebellion turned into a revolution that ended up destroying the old dynasty and set the stage for the Han period.

The Han dynasty played a very important role in Chinese history. They were the creators of Chinese history as we know it today. Additionally, the overwhelmingly predominant ethnic group in China nowadays is called the Han; named after the dynasty. More importantly, they developed (actually, it was invented by Qin Shihuangdi, but perfected by the Han) the administrative model which every successive dynasty would emulate in its entirety. The development of bureaucracy is crucial because ancient China was also a big country. In 206 BC, when the Han dynasty was founded, China stretched from modern Shenyang (some 500 km north of Beijing) in the north to around Guilin in the south; from the Pacific in the east to well past Chongqing in the west. Until Russia laid claim to Far East Siberia, China was the largest country in the world. It was also the most populous (60 million people at the time), and still is (however, India will probably overtake China in terms of population some time early in the 21th century). This is a management issue of tremendous proportions. How are you going to do things like collect taxes, keep the peace, and basically run a government without bureaucracy? The Chinese bureaucratic system is based on the study of the Confucian Classics, which provide an ideological reference point for appropriate behavior (which survived in spite of often being ignored) and loyalty to the Emperor. By developing this system, the Han emperors were able to run China with a reasonable degree of efficiency.
During the reign of an emperor named Han Wudi lived a historian named Sima Qian. His most important contribution to Chinese history was that he wrote a book known as Records of the Grand Historian which he claimed to be completing the book that his father, Sima Tan, had started but most of the book is Sima Qian's work. Most history books are very linear: first you talk about the Greeks, then the Romans, then the Dark Ages, and so on. What Sima did was structure his book so that each chapter covered a different topic: one chapter was a political record of the kings and emperors; the next would cover literature; the third, philosophy, and so on. Every dynastic record that followed copied Sima's original. Actually, there is an English-language history of China that loosely follows this model; called China's Imperial Past, written by Charles O. Hucker.

Between AD 8 and 25, a man named Wang Mang ruled China. He had been part of the Han royal household though he himself was a commoner and had no royal blood in his veins. He had been appointed emperor after a power struggle in the House of Han. His legacy is speckled because while he did seem to have done some good, reform-oriented ideas (such as. power back to the people), he really wasn't capable of ruling. After his death in AD 25, the Han royal family took back the reins of power, and set up the Later Han dynasty.

The later Han were able to keep intact for about another 200 years; however, towards the end of their rule, they become more and more dissipated. More importantly, they were unable to deal with two factors: a population shift from the Yellow River in the north to the Yangzi in the south; and they simply could not control barbarian tribal raiders from the north, which were one reason why people were moving to the south. Eventually, in AD 220, the center had lost so much control to the provinces that it collapsed (fueled by a small rebellion in the north), plunging China into 350 years of chaos and disunity.