The Zhou created the Mandate of Heaven: the idea that there could be only one legitimate ruler of China at a time, and that this ruler had the blessing of the gods. They used this Mandate to justify their overthrow of the Shang, and their subsequent rule.
Some scholars think the earlier Xia Dynasty never existed—that it was invented by the Zhou to support their claim under the Mandate that there had always been only one ruler of China.
The Chinese philosophical concept of the circumstances under which a ruler is allowed to rule. Good rulers were allowed to rule under the Mandate of Heaven, while despotic, unjust rulers had the Mandate revoked.
The Fall of the Shang
In 1046 BCE, the Zhou, a subject people living in the western part of the kingdom, overthrew the Shang Dynasty at the Battle of Muye. This was a battle between Shang and Zhou clans, over the Shang's expansion. They largely had the support of the Chinese people: Di Xin (the final king of the Shang Dynasty) had become cruel, spent state money on drinking and gambling, and ignored the state. The Zhou established authority by forging alliances with regional nobles, and founded their new dynasty with its capital at Fenghao (near present-day Xi'an, in western China).
Under the Zhou Dynasty, China moved away from worship of Shangdi ("Celestial Lord") in favor of worship of Tian ("heaven"), and they created the Mandate of Heaven. According to this idea, there could be only one legitimate ruler of China at a time, and this ruler reigned as the "Son of Heaven" with the approval of the gods. If a king ruled unfairly he could lose this approval, which would result in his downfall. Overthrow, natural disasters, and famine were taken as a sign that the ruler had lost the Mandate of Heaven.
The Mandate of Heaven did not require a ruler to be of noble birth, and had no time limitations. Instead, rulers were expected to be good and just in order to keep the Mandate. The Zhou claimed that their rule was justified by the Mandate of Heaven. In other words, the Zhou believed that the Shang kings had become immoral with their excessive drinking, luxuriant living, and cruelty, and so had lost their mandate. The gods' blessing was given instead to the new ruler under the Zhou Dynasty, which would rule China for the next 800 years.
The need for the Zhou to create a history of a unified China is also why some scholars think the Xia Dynasty may have been an invention of the Zhou. The Zhou needed to erase the various small states of prehistoric China from history, and replace them with the monocratic Xia Dynasty in order for their Mandate of Heaven to seem valid (i.e., to support the claim that there always would be, and always had been, only one ruler of China).
The Zhou ruled until 256 BCE, when the state of Qin captured Chengzhou. However, the Mandate of Heaven philosophy carried on throughout ancient China.