The Chou or Zhou dynasty (1050-256 BC) overthrew the Shang dynasty in 1050 BC. Chou dynasty took advantage the weak Shang rulers and united its enemies against it. The explanation of the Chou rulers for this usurpation of power was the concept of mandate of heaven.
Chou rulers understood the fact that it was impossible to hold on to power without proper explanation. Therefore, they found a new way to convince the nobles and people about the legitimacy of their rule. This new invention was the concept of the mandate of Heaven. Chou dynastic rulers explained that Shang dynasty lost the favor of the gods or mandate of heaven. The overthrow was successful only because of the lost mandate of Shang rulers. This concept became deep rooted in the Chinese politics and later became an integral part of Chinese society.
According to the new theory, a king or dynasty cannot rule without the support of gods. If the ruling dynasty lost the favor of the gods, they would loose the legitimacy to be in power.
The unhappiness of gods could interpreted with the different signs. Some of the signs were natural disasters, political upheavals, increased threats and disturbances etc. Under such conditions, a new government or emperor would take the power.