to Liang Province when Dong Zhuo was assassinated by Lü Bu. Along with his comrades, Fan Chou, Li Jue and Zhang Ji, Guo Si initially wanted to make Niu Fu their leader, but the latter was so scared and listened to his subordinate, Huchi'er (), to flee with the treasures he had saved. However, Huchi'er murdered Niu Fu after he decided to take the wealth of his master for himself. Guo Si, along the other three generals, then pleaded with the de facto leader of the Han central government, Wang Yun, for an amnesty since they were Dong Zhuo's most trusted aides. Instead, Wang Yun granted amnesty to all of Dong Zhuo's former subordinates except for these four. So they planned to relinquish their positions and go into hiding. Jia Xu suggested that they should take the opportunity to launch a strike at Chang'an since the Liang Province forces' power base was still intact after the coup. At the beginning of their campaign, few Liang Province residents joined them, only several thousands soldiers followed the four, but the number grew along the way. Wang Yun sent Xu Rong and Hu Zhen to fight the Liang Province forces en route at An'feng, but Xu Rong was killed in the first encounter and Hu Zhen persuaded his troops to join the rebels thus greatly increasing the size of the rebel force. A fierce battle was fought outside the walls of Chang'an, during which Guo Si personally duelled Lü Bu. Guo Si was defeated after being pierced by Lü Bu's spear. But he survived the wound and the Liang Province army pushed Lü Bu back into the city. The four then surrounded Chang'an with their army and suggested that they would leave if Wang Yun would go with them. Wang Yun, thinking that the four rebels only wanted his life and did not have the intention of conquering the city, committed suicide. After Wang Yun's death, the rebels managed to take control of Chang'an and secured power for themselves within the Han central government by taking Emperor Xian hostage and taking control over the selection of imperial officials for the emperor. As a regent The four then demanded high ranks, and Guo Si and Li Jue even thought about assassinating the emperor to take the throne, however was dissuaded from doing so by Fan Chou and Zhang Ji. With Li Jue as the