Towards the end of the 2nd millennium BC, the Shang were overrun by the Zhou dynasty from the Wei River valley to the west. The death of King Wu of Zhou soon after the conquest triggered a succession crisis and civil war that was suppressed by Wu's brother, the Duke of Zhou, acting as regent. The Zhou rulers at this time invoked the concept of the Mandate of Heaven to legitimize their rule, a concept that would be influential for almost every successive dynasty. The Zhou initially established their capital in the west near modern Xi'an, near the Yellow River, but they would preside over a series of expansions into the Yangtze River valley. This would be the first of many population migrations from north to south in Chinese history.