In 230 BCE, the Qin state annexed Han, the smallest of the six warring states. Located in the Central Plains, Han had the smallest territory and population among the six, surrounded by Wei, Qi, Chu, and Qin with no room for expansion, making it impossible to rival Qin's power, so Qin first set its sights on Han. King Yingzheng of Qin sent Neishi Teng to launch a surprise attack on Han, easily breaching its capital, Zhengcheng (present-day Xinzheng, Henan). King An of Han, knowing he could not resist, chose to surrender, leading to Han's downfall. King Yingzheng established Yingchuan Commandery in Han, with its seat at Yangdi (present-day Yuzhou, Henan).
The following year, a drought swept across the entire state of Zhao, and the King of Qin saw this as a perfect opportunity, immediately sending his troops in two separate forces to attack Zhao—one from the south and one from the north—both advancing fiercely toward the Zhao capital of Handan. In 228 BCE, Handan was captured by Qin forces, and the state of Zhao was destroyed. The King of Zhao was taken prisoner to Qin, while Prince Jia narrowly escaped and fled to Dai City (in present-day northeastern Weizhou, Hebei), where he gathered the remaining troops, declared himself the King of Dai, and awaited an opportunity for revenge.
In 226 BCE, King Ying Zheng of Qin set his sights on the State of Yan, harboring deep resentment toward Yan ever since the assassination attempt by Jing Ke. He dispatched the great general Wang Jian to lead a campaign against Yan, and Crown Prince Dan of Yan personally commanded the army to resist. However, the Yan forces were no match for the Qin army and quickly collapsed, with many killed or fleeing. King Xi of Yan and Crown Prince Dan fled to Liaodong to hide. In an attempt to appease the King of Qin's anger, King Xi of Yan had Crown Prince Dan executed.
Now Korea had been incorporated into the territory of the Qin state, leaving Zhao with only the city of Dai, and Yan with only a small area around Liaodong. It was impossible for them to rebuild their states. King Ying Zheng of Qin decided not to rush into attacking Zhao and Yan in the north, but instead to first conquer Wei and Chu in the south.
In 225 BC, Qin King Ying Zheng sent Wang Ben, the son of Wang Jian, to lead troops out of the Guanzhong region. Wang Ben captured over a dozen cities in northern Chu with little effort, establishing a solid base for attacking the state of Wei. He then marched his army north to assault Daliang (present-day Kaifeng, Henan), the capital of Wei. However, Daliang's fortifications were exceptionally strong, and no matter how hard they tried, they could not breach the walls. After much deliberation, Wang Ben devised a clever plan. He ordered a canal to be dug from the Yellow River all the way to Daliang, channeling the river's water to the base of the city walls. Over three months, the constant soaking gradually softened the walls, and the erosion at the base caused them to collapse under their own weight. Daliang fell instantly to the Qin forces, and King Jia of Wei was forced to surrender, becoming a prisoner taken to Qin.
While attacking the state of Wei, King Ying Zheng of Qin also sent Li Xin with 200,000 troops to attack the state of Chu. Li Xin quickly captured Pingxing (southeast of present-day Runan, Henan) and Qin (southeast of present-day Shenqiu, Henan), but he grew overly arrogant and underestimated the enemy. Later, he was thoroughly defeated by the Chu general Xiang Yan, and Li Xin retreated step by step, eventually fleeing back to Qin in haste.
King Ying Zheng of Qin was furious and dispatched the veteran general Wang Jian, ordering him to lead an army of 600,000 men to conquer the state of Chu—this was nearly the entire military force of Qin. When they reached Mount Tianzhong (present-day northwest of Shangshui, Henan), Wang Jian ordered the army to halt and set up camp, without giving any orders to engage in battle, and they simply stayed there. The Chu general Xiang Yan was puzzled and launched several probing attacks, but Wang Jian refused to come out and fight, continuing with his daily routines as usual. A year passed like this, and the Chu army let their guard down, believing that the Qin forces were merely stationed there and not intent on fighting. So Xiang Yan led his troops to prepare to return to the capital. Seeing the Chu army ready to retreat, Wang Jian immediately ordered the Qin forces to launch a lightning-fast attack. The Chu soldiers panicked, caught off guard by this sudden move, realizing that everything before had been a ruse. Xiang Yan fought desperately in the chaos, but the Qin troops were high in morale and pressed the Chu army back relentlessly. The two sides clashed fiercely at Qicheng, where the Chu forces suffered heavy losses. Then Wang Jian pressed on with momentum, crossed the Huai River, and advanced all the way to Shouchun.
In 223 BC, Shouchun was captured by the Qin army, the King of Chu was taken prisoner, and Xiang Yan, along with the escaped Lord Changping (the king's brother), fled to the region south of the Yangtze River, where they supported Lord Changping as the new King of Chu, intending to mount a stubborn resistance against Qin. After the fall of Chu, the Qin state established Jiujiang Commandery within its territory.
Wang Jian decided to completely eliminate the remaining forces of the Chu state. He first ordered the construction of ships, then trained a naval force. Once the ships were built, he led the navy across the Yangtze River and continued his offensive, successfully killing Xiang Yan and Lord Changping. In 222 BC, Wang Jian captured the city of Kuaiji in southern Chu, and King Ying Zheng of Qin subsequently established Kuaiji Commandery in that region.
In this same year, Wang Ben led his army to capture Liaodong, then attacked Dai City; King Xi of Yan and King Jia of Dai were both taken prisoner and brought back to Qin.
Before deciding to annex the six other states, King Ying Zheng of Qin used a scheme to win over Hou Sheng, the prime minister of Qi, to his side. He wanted Qi to submit to him and refuse any requests for aid from other states. Hou Sheng then whispered in the ear of the King of Qi, who, after some thought, did exactly as the King of Qin instructed, believing that the only way to preserve Qi was to rely on Qin. He thought that by obeying every command of the King of Qin, he could avoid the fate of national destruction—but he was wrong.
It was not until the other five states had all been annexed by the State of Qin that the King of Qi finally realized the truth, and by then Qin had also revealed its intention to swallow Qi. The King of Qi hastily sent troops to guard the western borders, fantasizing that he might be able to resist Qin's attack, but suddenly he realized that all the other states had been destroyed and there was no one left to help him. Only then did he understand the ruthlessness of the King of Qin, and he could not help but feel deep regret.
In 221 BCE, King Ying Zheng of Qin planned to attack the state of Qi. He decided to come up with a decent excuse as a pretext for sending out his troops, especially since Qi had always been obedient to Qin before. Soon, Wang Ben led a massive army marching toward Qi, accusing the Qi king of neglecting the Qin envoys, and then launched an assault. Poor Qi's army, having not fought in years and intimidated by the Qin forces' momentum, had no strength left to resist. Within just a few days, the capital Linzi was breached by the Qin army, and the Qi king had no choice but to surrender.
The State of Qi was destroyed. After more than 200 years of effort, by the time of King Ying Zheng of Qin, the unification of the Chinese land was finally achieved, marking an inevitable trend of social development and opening a new chapter in history.