Hello friend...
The Zhou dynasty was pretty much non existent after the sacking of the capital by Quanrong in 771 BC since after that Zhou dynasty could no longer exert its power on its vassal states anymore. The vassal states now instead of listening to Zhou king they would listen to a Duke from a major state like Duke Huan of Qi (reign, 685 -643 BC), Duke Wen of Jin (reign, 636–628 BC), King Zhuang of Chu (reign, 613–591 BC), Duke Mu of Qin (reign, 659–621 BC). Some states outright countered the power of Zhou like Duke Zhuang of Zheng (743–701 BC) who even attacked Zhou. In Spring and Autumn period (770–475 BC) those Dukes still borrowed the name of Zhou king for their own advantage, so sometimes they paid tribute to Zhou king to borrow his approval. That was just to be nice, if Zhou king disapproved then the one who lost more was Zhou king himself since he would no longer receive any things from that major state and all other states allied to that major state. From Warring States period (475–221 BC), no one cared about what Zhou king had to say anymore and no one bothered to get his approval anymore. They just freely fought each other, conquered each other and deemed Zhou king as just lord of a minor state that they can take over anytime they wanted. Eventually, the little land of Zhou was conquered by Qin in 2 occasions: 256 BC and 249 BC.
Hence, officially, Zhou ended in 249 BC by the invasions from Qin. However, its power as a proper dynasty would have ended long before in the year of 771 BC.
And why the Zhou was attacked and lost its power in 771 BC ? That was largely due to the poor governing and poor credibility of King You of Zhou (reign 782–771 BC). He was a bad King, just like King Zhou of Shang, only into wines and girls without giving any thought on governing. In order to buy a laughter from the girl he loved, he ignited the big flames in the mountains which were used very exclusively for the cases of a serious emergency. Soldiers from vassal states were supposed to march toward the Zhou capital whenever they saw those flames, believing that the Zhou king was in danger. Thus, when they saw the king and his lover ridiculed them coming for nothing, they thought they were humilated by the king, and the king who played on the fate of his country didn’t deserve his post. Since then, the confidence that vassal states had on the king was all gone. And when Qianrung attacked the Zhou capital, no one came rescue when the flames were lit again. The king was killed in the attack, and Zhou lost its power since then. So I think the fall of Zhou was mainly due to King You of Zhou himself (due to his incompetence and moral depravity) since before him his father King Xuan of Zhou (827- 782) was a capable ruler, who brought back the stability and prosperity of Zhou.
And the further reason might be due to its problematic way of governing when at the foundation, Zhou rulers gave so much autonomy for their vassal states. Thus, when the central government became weakened and at the same time vassal states got stronger, inevitably Zhou central government can no longer hold its vassal states together. Later dynasty, Qin, Han got the lessons from Zhou and chose to build a centralized government with emperor having absolute power over any governing branches and any jurisdiction.
Hope this helps you...
