This was completed in 221 BC with the fall of Qi, which further led to a more centralised form of government replacing the fengjian system of the Zhou dynasty.

After the last "horizontal alliance" to punish Qi ended in 284, Qin fought multiple wars against Zhao in 283, 269 and 265 BC.

In 238 BC, King Zhuangxiang's son Ying Zheng took the reins of power in Qin 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 states and unifying China.

Its key steps were to ally with Yan and Qi, deter Wei and Chu, and conquer Han and Zhao.

In 230 BC, the Qin army led by Neishi Teng (內史騰)[a] moved south, crossed the Yellow River, and conquered the Han capital Zheng (鄭; present-day Xinzheng, Henan) within one year.

The Qin army led by Wang Jian conquered the Zhao territories of Eyu (閼與; present-day Heshun County, Shanxi) and Liaoyang (撩陽; present-day Zuoquan County, Shanxi), while the other Qin army under the command of Huan Yi, Qiang Lei (羌瘣) and Yang Duanhe (楊端和) captured Ye and Anyang.

Two years later, Qin planned to attack Han but feared that Zhao might support Han, so Huan Yi was ordered to lead an army from Shangdang to attack the Zhao territories of Pingyang (平陽) and Wucheng (武城), both located south of present-day Ci County, Hebei.

[2] In 233 BC, Huan Yi crossed the Taihang Mountains and conquered the Zhao territories of Chili (赤麗) and Yi'an (宜安), both located southeast of present-day Shijiazhuang, Hebei.

Meanwhile, the Qin general Li Xin led two forces from Taiyuan and Yunzhong to attack Dai County in the north.

In 228 BC, after learning that Li Mu had been eliminated, the Qin forces attacked and conquered Dongyang (東陽; located east of the Taihang Mountains).

In 222 BC, Wang Ben led Qin forces to conquer Dai and captured Prince Jia.

In 226 BC, Ying Zheng used the assassination attempt as a casus belli to order Wang Jian and Meng Wu to lead Qin forces to attack Yan.

King Xi, Crown Prince Dan, and the surviving Yan forces retreated to the Liaodong Peninsula.

In order to make peace, King Xi executed Crown Prince Dan and sent his head to Ying Zheng as an "apology" for the assassination attempt.

[3] The former Yan territories were partitioned and reorganised to form the Yuyang, Beiping, Liaoxi and Liaodong commanderies of the Qin Empire.

Wang Ben then led his forces north to attack and besiege the Wei capital Daliang (大梁; northwest of present-day Kaifeng, Henan).

As Daliang was situated at the concourse of the Sui and Ying rivers and the Hong Canal (鴻溝; in present-day Zhengzhou, Henan), its geographical location gave it a natural defensive advantage.

His troops laboured for three months to redirect the water flow while maintaining the siege on Daliang, and succeeded in their plan.

In 226 BC, Qin forces led by Wang Ben had attacked Chu and conquered ten cities.

Ying Zheng chose to follow Li Xin's idea, ordering him and Meng Tian to lead 200,000 troops to attack Chu.

Upon learning of Li Xin's defeat, Ying Zheng visited Wang Jian, apologised for not heeding his advice earlier, and invited him back to serve in the Qin army.

Wang Jian knew that Ying Zheng did not fully trust him because he could easily turn against Qin with such a massive army under his command.

The following year, Wang Jian and Meng Wu led Qin forces to attack the Wuyue region (covering present-day Zhejiang and Jiangsu), which was inhabited by the Baiyue, and captured the descendants of the royal family of the ancient state of Yue.

Even though their troops were not well-equipped and morale was low, the Qi government hurriedly mobilised them to the western border to counter a Qin invasion.

In the south, a Qin army of 500,000 attacked Yue and subjugated the Baiyue peoples who inhabited the areas around present-day Jiangsu, Zhejiang, Fujian, and Guangdong.

Some of the victorious rebels claimed to be restoring the former states that had been conquered by Qin, and numerous pretenders to the vacant thrones began to emerge.