Qin dynasty
Note: King Zhaoxiang of Qin (秦昭襄王) had already been ruling Qin for 51 years when Qin annihilated the Zhou dynasty; however the other six warring states were still independent regimes. Some Chinese historiographers thus used the next year (the 52nd year of King Zhaoxiang of Qin) as the official succession from the Zhou dynasty.
History of the Han dynasty
The Zhou dynasty (c. 1050–256 BCE) had made the State of Qin in Western China as an outpost to breed horses and act as a defensive buffer against nomadic armies of the Rong, Qiang, and Di peoples. After conquering six Warring States (i.e. Han, Zhao, Wei, Chu, Yan, and Qi) by 221 BCE, the King of Qin, Ying Zheng, unified China under one empire divided into 36 centrally-controlled commanderies. With control over much of China proper, he affirmed his enhanced prestige by taking the unprecedented title "huangdi" (皇帝), or 'emperor', known thereafter as Qin Shi Huang (i.e. the first emperor of Qin). Han-era historians would accuse his regime of employing ruthless methods to preserve his rule.
Qin Shi Huang died of natural causes in 210 BCE. In 209 BCE the conscription officers Chen Sheng and Wu Guang, leading 900 conscripts through the rain, failed to meet an arrival deadline; the "Standard Histories" claim that the Qin punishment for this delay would have been execution. To avoid this, Chen and Wu started a rebellion against Qin, known as the Dazexiang Uprising, but they were thwarted by the Qin general Zhang Han in 208 BCE; both Wu and Chen were subsequently assassinated by their own soldiers. Yet by this point others had rebelled, among them Xiang Yu (d. 202 BCE) and his uncle Xiang Liang (項梁/项梁), men from a leading family of the Chu aristocracy. They were joined by Liu Bang, a man of peasant origin and supervisor of convicts in Pei County. Mi Xin, grandson of King Huai I of Chu, was declared King Huai II of Chu at his powerbase of Pengcheng (modern Xuzhou) with the support of the Xiangs, while other kingdoms soon formed in opposition to Qin. Despite this, in 208 BCE Xiang Liang was killed in a battle with Zhang Han, who subsequently attacked Zhao Xie the King of Zhao at his capital of Handan, forcing him to flee to Julu, which Zhang put under siege. However, the new kingdoms of Chu, Yan, and Qi came to Zhao's aid; Xiang Yu defeated Zhang at Julu and in 207 BCE forced Zhang to surrender.