After King Huai of Chu was made the Righteous Emperor, Xiang Yu arranged for him to be relocated to Changsha in Hunan, but secretly sent Ying Bu to assassinate him along the way, and from then on, state power was completely in the hands of Xiang Yu, the Hegemon-King of Western Chu.
To eliminate future threats, Xiang Yu assigned to Liu Bang the sparsely populated and harsh region of Ba-Shu and Hanzhong, and stationed three surrendered Qin generals around Liu Bang to monitor his movements at all times. Zhang Han was one of these three, and Xiang Yu deliberately placed him in the most critical position—if Liu Bang ever attempted to break out through the passes, Zhang Han's territory would be the inevitable route.
Liu Bang, the King of Han, knew his forces were no match for Xiang Yu's. Although he had been the first to enter the Guanzhong region, he was assigned to the farthest territory. Liu Bang could only vent his frustration inwardly, but outwardly he followed Xiang Yu's arrangements and went to the Bashu region. The only passage into Bashu was a plank road built along cliffs. After all of Liu Bang's troops had passed through, Zhang Liang advised Liu Bang to destroy this sole route, arguing that it would not only show they had no ambition to seize the realm but also prevent Xiang Yu from pursuing and annihilating them. Liu Bang agreed and ordered the plank road to be burned to ashes.
The news of the plank road being burned soon reached Zhang Han, who indeed fell for the trick, thinking that Liu Bang was truly content to stay in that remote place, which significantly lowered Zhang Han's vigilance. However, Liu Bang's generals and soldiers were also in despair, unaware of Pei Gong and Zhang Liang's plans, believing they would be stuck in Ba and Shu for the rest of their lives, leaving everyone feeling hopeless. After Han King Liu Bang settled down in Ba and Shu, those longing to return home seized every opportunity to escape, with several people running away almost every night. Liu Bang grew anxious, realizing that if this continued, his army would soon be completely gone.
Just then, Xiahou Ying recommended an unremarkable soldier to Xiao He, saying that this man was a rare talent who should be valued. This soldier was named Han Xin, born a commoner with a straightforward and unrestrained personality. In his hometown, he was so impoverished that he often went without food and couldn't even afford a proper burial for his mother, yet he persisted in reading countless books. A local village chief, seeing Han Xin's strong will and believing he would achieve great things, invited him to stay at his home. However, the chief's wife despised Han Xin, and in anger, he left. He was saved from starvation only by an old woman washing silk by the river who gave him food. Later, a young butcher tried to humiliate Han Xin, demanding that he crawl between his legs or fight him. Unwilling to entangle himself with such a reckless man, Han Xin slowly crawled under the butcher, earning ridicule from many neighbors. When the rebellion against the Qin Dynasty erupted across the country, Han Xin left home to join Xiang Yu. He repeatedly offered military advice to Xiang Yu, but Xiang Yu dismissed him. Coupled with Xiang Yu's unpopular actions after entering Xianyang, Han Xin concluded that Xiang Yu could never become a true ruler, so he abandoned Chu to join Han, following Liu Bang to his fiefdom. Han Xin received no recognition in Liu Bang's army either, only a minor position guarding a warehouse. Unfortunately, he violated military law and was sentenced to death along with a dozen others. At the critical moment, Han Xin saw Xiahou Ying passing by and shouted to him, "Isn't the King of Han trying to win the world? Why kill a man with strategy?" Startled by this, Xiahou Ying was amazed that an ordinary soldier had revealed a secret known only to high-ranking leaders. He looked at Han Xin, saw his courage and intelligence, and recognized his exceptional qualities, thus saving his life.
After chatting with Han Xin for a few moments, Xiao He discovered that he was indeed highly learned and resourceful, so he recommended Han Xin to Liu Bang. However, Liu Bang, like Xiang Yu, felt that Han Xin had no military achievements and would be hard to convince others to accept. Seeing the King of Han's attitude, Han Xin became utterly disheartened. Watching people around him constantly desert, Han Xin also decided to leave Bashu.
That night, as Han Xin was leaving on horseback, he hadn't gone far when he heard Xiao He calling out from behind. It turned out that Xiao He, upon realizing Han Xin had left, had ridden hard to catch up and, after much persuasion, managed to convince him to return. This time, Xiao He solemnly told Liu Bang that if he wanted to conquer the realm, Han Xin was indispensable. Xiao He also requested that Liu Bang appoint Han Xin as Grand General, giving him command over all military operations. So Liu Bang set up an altar, fasted and bathed, and formally enfeoffed Han Xin as Grand General before the entire army. The troops were astonished but had no choice but to obey. From then on, with the help of Xiao He and Han Xin, Liu Bang steadily advanced eastward, contending with Xiang Yu for control of the realm.
Liu Bang first asked Han Xin for a good strategy to contend with Xiang Yu, but Han Xin told him that in terms of sheer strength, Xiang Yu was undoubtedly number one, and no one dared to disobey his commands. However, Xiang Yu lacked strategic wisdom, relying only on brute force in battle. Although he had a merciful heart, he was indecisive and suspicious by nature. Every time his army marched through a region, they would burn, kill, and loot, losing the people's support. Moreover, he had violated the agreement with King Huai of Chu, and Han Xin believed that few people under heaven were willing to submit to him, predicting that rebellion would soon arise. Han Xin then said that when Liu Bang, then known as the Duke of Pei, entered the Guanzhong region, he was considerate and loved the people, winning the support of all the Qin commoners. The title of King of Guanzhong should have rightfully gone to Liu Bang, but Xiang Yu had seized it, and the people felt it was unjust. If Liu Bang raised an army, the populace would surely side with him. Among Xiang Yu's subordinates were three Qin generals who had surrendered, but when they led hundreds of thousands of Qin troops, Xiang Yu had them all buried alive, so these three generals harbored resentment. Han Xin suggested starting with these three Qin generals—whether by forming an alliance or eliminating them—as seizing their three fiefs would greatly increase the chances of victory.
After hearing Han Xin's words, Liu Bang was overjoyed, only regretting that he had not promoted him sooner. Han Xin further suggested that Liu Bang should, under the pretext of repairing the plank road, first send a small portion of troops to divert the attention of the three generals, then lead the main army out from a concealed location to launch a surprise attack on them.
In the second month of the first year of the Han Dynasty (206 BCE), Liu Bang sent Fan Kuai to lead a large force to repair the plank roads, then personally led his main army across the Qinling Mountains, completely surrounding Zhang Han, who only then realized Liu Bang's true intentions, but it was too late; despite fierce resistance, Chencang was captured by the Han army, and Zhang Han fled hastily with his remaining troops, only to be completely annihilated later, after which the Han army pressed on to conquer the territories of the other two Qin generals, officially marking the beginning of the war between Liu Bang and Xiang Yu.