In October of the first year of Hanwang (206 BC), Liu Bang became king in Guanzhong after the Qin Dynasty was destroyed. In December, Xiang Yu invaded Hanguguan to destroy Liu Bangjun. Knowing the defeat, Liu Bang went to Hong Men (now northeast of Lintong, Shaanxi) to apologize.
Soon, Xiang Yu entered Xianyang. In the spring, Xiang Yu regarded Chu Huaiwang as the righteous emperor on the surface, but actually sent him to the south of the Yangtze River, where he became the overlord of the land of Xi, the capital of Pengcheng (now Xuzhou, Jiangsu Province), and at the same time enfeoffed 18 vassal, named Liu Bang as Hanwang, and commanded Bashu and Hanzhong, and deliberately appointed Qin generals Zhang Han, Sima Xin and Dong Kun as Wang Sai and Zhaiwang.
Liu bang had to submit to humiliation and accept the title. In April, he led the troops into Hanzhong and burned the plank road (the road paved with boards on the cliff), indicating that he had no intention of sending troops to paralyze Xiang Yu again. Xiang Yu also returned to the East with a great army. In May and June, Tian Rong, a descendant of Qi nobles, was dissatisfied with the enfeoffment, drove away the King of Qi, killed the King of Jiaodong, and established himself as the King of Qi.
Liu Bang took advantage of the chaos to return to Guanzhong, defeated Zhang Han, forced Sima Xin and Dong Kun to land, and tricked Xiang Yu into believing that he was satisfied with Guanzhong and would never move eastward again. Xiang Yu safely attacked Tian Rong, and did not strengthen the prevention of the West. In October, Liu Bangdong invaded, worshipped Han Xin as a general, built a plank road, and secretly crossed Chencang (now east of Baoji City, Shaanxi Province), saying that the nominal emperor sent someone to contact the governors and publicly denounced Xiang Yu, which opened the prelude to the Chu-Han War.
The First Battle of Pengcheng: In April, Liu Bang took advantage of Xiang Yu's stay in Qi to lead the vassal army to capture Pengcheng in one fell swoop. Hearing this, Xiang Yu quickly led 30,000 chosen men to attack and annihilate more than 200,000 allied troops. Liu bang only led dozens of riders to escape, and the anti-Chu alliance collapsed.
Fight for: In May, Liu Bang arrived in Xingyang, repelled Chu's pursuers, stopped breathing, stabilized his position, reorganized the army, and relied on the Guanzhong base area and favorable terrain to fight against Xiang Yu for a long time. In June, Liu Bang sent troops to attack the waste hills and forced Zhang Han to commit suicide, which relieved his worries. Send someone to persuade Qing Bu to oppose Chu and contact Peng Yue to disturb Chu's rear; Han Xin was sent to open up the northern battlefield, attack Wei, destroy the generation, destroy Zhao and kill. In the winter of three years, Xiang Yu launched a counterattack and besieged Xingyang. The situation is very critical.
Liu Bang used Chen Ping's double spy to make Xiang Yu doubt Fan Zeng, and forced Fan Zeng to return home in a rage. Liu Bang also sent Ji Xin to pretend that he had surrendered to the Chu army and took the opportunity to escape from Xingyang. Xiang Yu stepped up the siege of Xingyang and occupied the elevation.