Liu Bei's takeover of Yi Province

Liu Bei led an expedition force into Yi Province after leaving behind Zhuge Liang, Guan Yu, Zhang Fei and Zhao Yun to guard Jing Province. Liu Zhang received Liu Bei warmly at Fu County (涪縣; in present-day Fuling District, Chongqing) and provided him with more troops under his command as well as abundant provisions and equipment. Liu Bei headed to Jiameng Pass (southwest of present-day Guangyuan, Sichuan) at the border between Liu Zhang and Zhang Lu's territories. Instead of engaging Zhang Lu, Liu Bei halted his advance and focused on building up connections and gaining influence around the area in preparation for his takeover of Yi Province.[3]

In 212, Pang Tong outlined three plans for Liu Bei to choose from: The first was to advance swiftly to seize Chengdu, the capital of Yi Province, from Liu Zhang. The second was to take command of Liu Zhang's armies in the north and then move to capture Chengdu. The third one was to return to Baidicheng to await further action. Liu Bei chose the second option and lied to Liu Zhang that he needed more troops to divert Cao Cao's attention away from the east (where Sun Quan was under attack), and requested for another 10,000 soldiers and additional provisions to aid in the defense of Jing Province. Liu Zhang gave him 40% of the troops and half of the others he asked for.

Zhang Su (張肅), Zhang Song's brother, discovered his brother's secret communications with Liu Bei and reported the issue to Liu Zhang. Liu Zhang was furious and stunned when he heard that Zhang Song had been helping Liu Bei to plot against him—he executed Zhang Song and ordered his military officers guarding the passes to Chengdu to keep secret his knowledge of Liu Bei's intention. Despite Liu Zhang's efforts, Liu Bei still found out from the spies he planted around Liu Zhang. Before Liu Zhang's men could reach Yang Huai (楊懷) and Gao Pei (高沛), the military officers guarding Boshui Pass, Liu Bei summoned and killed them on the excuse that they were disrespectful towards him. Liu Bei then took over their troops and turned to attack Fu County (涪縣; in present-day Fuling District, Chongqing). On his way, Liu Bei's troops bypassed Zitong Commandery, where the defenders closed the gates and resisted the invasion.[4]

In the spring of 213, Liu Zhang sent Liu Gui (劉璝), Leng Bao (冷苞), Zhang Ren, Deng Xian (鄧賢) and other military officers under Wu Yi to defend Mianzhu (綿竹). Despite being the most trusted vassal of Liu Zhang, Wu Yi soon changed allegiance, so Li Yan and Fei Guan (費觀) were sent to replace him, but they surrendered to Liu Bei as well. At the time, Liu Zhang's soldiers were not used to intense combat and were low on morale, and he did not dish out intimidating punishments to the families of those surrendered, so only a few generals were determined to fight the invaders. On the contrary, Liu Bei was promising to provide wealth and positions to those who would switch allegiance, so many of Liu Zhang's subordinates simply welcomed him along his way until he reached Luo County (雒縣; north of present-day Guanghan, Sichuan), where the ones who could not be bought by wealth nor power played their staunch defence.

Liu Zhang's eldest son, Liu Xun (劉循), took command of the remnants and retreated to Luo County (雒縣; north of present-day Guanghan, Sichuan) to join forces with Ling Bao. Liu Bei ordered his troops to surround Luo County and sent his adviser Pang Tong to lead a major assault on the county. However, the defenders fought back with projectiles and Pang Tong was killed by a stray arrow. Thus, the siege became a prolonged one, forcing Liu Bei to call forth reinforcements from Jing Province. Zhang Fei led an army from Jing Province to attack Jiangzhou (in present-day Chongqing), where he captured an enemy officer, Yan Yan. Facing Zhang Fei's insults, Yan Yan condemned him for invading Yi Province. Yan Yan was originally ordered to be executed by the angry Zhang Fei, but his fearlessness of death impressed Zhang, who pardoned the captive's life. Following the Dian River (垫江), Zhang Fei broke through a thin defence line, guarded by an enemy major, and rendezvoused with Liu Bei. Zhao Yun and Zhuge Liang also arrived from another route.

One of Liu Zhang's officers, Zhang Ren, attempted to break the siege by leading his men onto the Wild Goose Bridge and charging Liu Bei's forces, but he was defeated and captured. His loyalty and bravery were well known, so Liu Bei ordered his army to force the captive into submission. However, Zhang Ren said, "A loyal subject will never serve two masters!" Liu Bei lamented his determination and had him executed.[5]

More than a year had passed and Luo County finally fell, but Liu Zhang continued to hold on inside Chengdu. Ma Chao, a former Liang Province warlord and a vassal of Zhang Lu, killed Zhang Lu's subordinate Yang Bai (楊柏) and joined Liu Bei. Upon seeing Ma Chao's army to the north of Chengdu, the citizens inside the city were terrified, but they still insisted on fighting a desperate war.[6] However, the morale of Liu Zhang's army was at an all-time low and only Huang Quan was insistent on resisting Liu Bei despite a losing effort. Liu Bei decided to send his longtime adviser, Jian Yong, whom Liu Zhang was already fond of before the war broke out, to speak to Liu Zhang. Jian Yong managed to convince Liu Zhang to surrender. Thus, Liu Zhang opened Chengdu's gates and surrendered to Liu Bei; everyone inside the city wept for Liu Zhang's sacrifice.[7] Liu Bei then succeeded Liu Zhang as Governor of Yi Province, and he conferred on the latter the seal and tassel of General of Vehement Might (奮威將軍) before expatriating him and Liu Xun to Gong'an County in Jing province.

Liu Bei married Wu Yi's sister and attempted to solidify his control on the newly conquered domain. Most of Liu Bei's followers, new and old, were promoted to higher ranks; and a set of straight law code was applied to Yi Province, replacing the softer one set by Liu Zhang.