A chieftain of a frontier tribe called Zhou, which had settled in the Wei Valley in modern Sha'anxi Province, overthrew the last Shang ruler, a despot according to standard Chinese accounts. The Zhou Dynasty lasted longer than any other, from 1027 to 221 BC (807 years). It was philosophers of this period who first enunciated the doctrine of the "mandate of heaven," the notion that the ruler (the "son of heaven") governed by divine right but his dethronement would mean that he had lost his mandate. This doctrine explained and justified the demise of the two earlier dynasties and at the same time supported the legitimacy of the present and future rulers. In 771 BC, the Zhou court was sacked, and its king killed by invading barbarians who were allied with rebel lords. The capital was moved eastward to Luoyang. Because of this shift, historians divide the Zhou era into Western Zhou (1027 to 771 BC) and Eastern Zhou (770 to 221 BC). With the royal line broken, the power of the Zhou court gradually diminished; the fragmentation of the kingdom accelerated. Eastern Zhou divides into two sub periods. The first, from 770 to 476 BC, is called the Spring and Autumn Period, after a famous historical chronicle of the time; the second is known as the Warring States Period (475 to 221 BC).