World History/The Unification of China

As you might have known, the early Chinese dynasties, namely Shang and Zhou, are not centralized countries, which means that the central government did not have direct power over the country. This is because the national power is still too weak at that point. To rule the land it couldn't directly rule, the Zhou dynasty developed a system called Fengjian, which is similar to Feudalism in Europe. The King of Zhou distributed land to its relatives, courtiers, or decedents of the former dynasties, making them constituent states called Zhuhou (諸侯).

The states were supposed to back up the Zhou court, but the power of the court had started to decrease since 770 BC. During the Spring and Autumn period (770 to 476 BC), the Zhou royal authority over the states eroded as more and more of them obtained de facto regional autonomy, defying the king's court and waging wars amongst themselves. Many small states, including various city states, were annexed by larger states, but the war during the period were mostly aimed at getting more vassal states than annexing other states.

This, however, gradually changed in the the Warring States period. The gradual Partition of Jin, one of the most powerful states, marked the end of the Spring and Autumn period and the beginning of the Warring States period (475 to 221 BC). The states had more large-scaled wars which were aimed at annexation. The state of Qin gradually became the dominant power of the states. It overthrew the court of Zhou, unified the remaining states, and in 221 BC formed the Qin dynasty.

Background
Even the Zhou Dynasty lasted for almost 900 years, its territory was divided into numerous states, most powerful, being (in alphabetical order): Chu, Han, Qi, Qin, Wei, Yan, and Zhao.

In 221 B.C.,Chinese were unified for the first time to construct a great country that ended the long era of disunity and warring. In that year the western frontier state of Qin, the most aggressive of the Warring States, subjugated the last of its rival state.