Qin's wars of unification were a series of military campaigns launched in the late 3rd century BC by the Qin state against the other six major Chinese states — Han, Zhao, Yan, Wei, Chu and Qi.
Between 247 BC and 221 BC, Qin had emerged as one of the most powerful of the Seven Warring States in China. In 230 BC, Ying Zheng, the King of Qin, unleashed the final campaigns of the Warring States period, setting out to conquer the remaining states one by one. Following the fall of Qi in 221 BC, China was unified under Qin control. Ying Zheng declared himself "Qin Shi Huang" (meaning "First Emperor of Qin") and established the Qin dynasty, becoming the first sovereign ruler of a unified China.
Background
Rise of Qin and early conquests
Over the course of the Warring States period, the Qin state had evolved to become the most powerful of the seven major states in China, particularly after Shang Yang's reforms in the mid 4th century BC. The other six states would repeatedly adopt the policy of "vertical alliance" or hezong (合縱) in which they all joined in as allies against Qin. Qin state itself sometimes allied with other states in a "horizontal alliance" or lianheng (連橫), waging numerous wars against each other.
In 316 BC, Qin expanded south towards the Sichuan Basin by conquering the states of Ba and Shu. In 278 BC, the Qin general Bai Qi attacked Chu from the new conquered land in Sichuan. The Chu capitals of Ying and Chen (陳) were captured and Chu lands west to the Han River were lost. In 272 BC, Qin conquered the Xirong state of Yiqu. After the last horizontal alliance to punish Qi ended in 284 BC, Qin began to wage multiple wars against the northern Zhao state in 283, 269 and 265 BC.
In 269 BC, Fan Sui became chief advisor to Qin. He advocated authoritarian reforms, irrevocable expansion and an alliance with distant states to attack nearby states. In 265 BC, King Zhaoxiang of Qin made the first move by attacking the Shangdang region of Han state. The governor of Shangdang refused to surrender and presented it to the King of Zhao. The Qin and Zhao armies were locked in a 2-year siege at Changping, which culminated in a decisive defeat of Zhao by the Qin army. In 257 BC, the Qin army failed to besiege the Zhao capital Handan and was defeated by the allied force of Zhao, Wei and Chu.
At the time of King Nan of Zhou, the kings of Zhou dynasty had lost almost all political and military power, their remaining crown land was split into two states: West Zhou, centered in Wangcheng, and East Zhou, centered at Chengzhou. Qin forces conquered West Zhou in 256 BC, claiming the Nine Cauldrons and thereby symbolically becoming The Son of Heaven. In 249 BC, the new Qin king Zhuangxiang conquered East Zhou, putting to end the 800-year Zhou dynasty.
First Emperor Ying Zheng
In 238 BC, Ying Zheng, the King of Qin, took over the reins of power after eliminating his political rivals Lü Buwei and Lao Ai. With help from Li Si, Wei Liao () and others, Ying Zheng formulated a plan for conquering the other six major states and unifying China. The plan, which focused on annexing each state individually, was based on "allying with distant states and attacking nearby ones", one of the Thirty-Six Stratagems. Its key steps were to ally with Yan and Qi, deter Wei and Chu, and conquer Han and Zhao.
Unification
Conquest of Zhao
From 283 to 257 BC, Qin and Zhao fought in bloody wars. Zhao's defeat by Qin at Changping in 260 BC has severely weakened the latter state.
In 236 BC, while Zhao was attacking Yan, Qin used the opportunity to send two separate forces to invade Zhao. The Qin army led by Wang Jian conquered the Zhao territories of Eyu (; present-day Heshun County, Shanxi) and Liaoyang (; present-day Zuoquan County, Shanxi), while the other Qin army under the command of Huan Yi, Qiang Lei and Yang Duanhe () captured Ye and Anyang. Zhao lost nine cities and its military prowess was weakened.
Two years later, Qin planned to attack Han but feared that Zhao might support Han, so the Qin general Huan Yi was ordered to lead an army to attack the Zhao territories of Pingyang (; southeast of present-day Ci County, Hebei) and Wucheng (; southwest of present-day Ci County, Hebei). More than 100,000 soldiers were killed in the battle. The Zhao army was defeated and its commander, Hu Zhe (), was killed in action. In 233 BC, Huan Yi's army crossed Mount Taihang and conquered the Zhao territories of Chili () and Yi'an (), both located southeast of present-day Shijiazhuang, Hebei. Then, the Qin army under Huan Yi engaged the Zhao army commanded by General Li Mu at the Battle of Fei in which the Qin sustained a massive defeat and nearly lost all their forces.