King Zhaoxiang of Qin
In 293 BC, East Zhou decided to ally itself with the states of Han and Wei, and sent troops to help attacking Qin. However, the combined forces of this alliance was destroyed at Yique by the young Qin general Bai Qi, with 240,000 men killed and their commander Gongsun Xi (公孫喜) captured and executed. After this loss, the East Zhou was unable to actively participate in future interstate struggles in any significant fashion.
In 256 BC, Duke Wu of West Zhou (西周武公) allied with other states to stop the Qin offensive on the Han city of Yangcheng. In retaliation, King Zhaoxiang of Qin sent general Jiu to invade West Zhou, successfully breaking into Wangcheng. The Duke Wu was taken to Xianyang to beg for mercy, conceding all his lands. Both King Nan and Duke Wu would die later that year, and the subsequent Duke of West Zhou, Duke Wen (西周文公), was exiled to Danhuju (憚狐聚). Because the Zhou royal court had fallen from power and King Nan had died without a successor, the Eastern Zhou Dynasty collapsed, ending 879 years of Zhou monarchy. The remaining East Zhou state was also conquered by Qin chancellor Lü Buwei seven years later in 249 BC during the reign of King Zhaoxiang's grandson King Zhuangxiang, after Duke Jing of East Zhou attempted to form an anti-Qin alliance with other states.
King Zhaoxiang of Qin
The authority of the Zhou court had been declining since the very beginning of the Eastern Zhou Dynasty. When King Ping of Zhou abandoned the old capital of Haojing and relocated east to Luoyi, the new capital's crown land (王畿) was significantly smaller and less developed compared to the old capital. The royal Zhou court, which had been humiliated by Haojing's sacking, became increasingly reliant on the support of the vassal states. King Ping's son, King Huan, later had a falling out with one of the strongest vassals, Duke Zhuang of Zheng. The young King Huan decided to assert his authority as the lord paramount and personally led an expedition in 707 BC to punish on the state of Zheng, but was badly defeated in the Battle of Xuge. King Huan himself was wounded in the shoulder by an arrow, and was forced to back down and negotiate peace with Duke Zhuang. This destroyed any remaining prestige and authority the Eastern Zhou royal court had over its vassals, and confirmed the "de facto" independence of the feudal states. The Zhou court was so impoverished after that defeat that, when King Huan died in 697 BC, it took the court seven years to get enough funding for a fitting royal funeral. During the reign of King Xiang of Zhou, his half-brother Prince Dai (王子帶) twice attempted to usurp the throne in 649 BC and 635 BC, both times by conspiring with Quanrong to lay siege to the capital. King Xiang required military assistance by the vassal states to reinstate himself on both occasions, and was forced to increase their fiefs each time as commendations. This further weakened the image of the Eastern Zhou court in the eyes of the vassal states, and progressively relegated it to the status of a neglected figurehead.