Gongsun Zan
In the 190s, Liu Yu and another warlord Gongsun Zan got into conflict because of opposing views on how to deal with the ethnic minority tribes and foreign kingdoms: Liu Yu adopted a peaceful and pacifist policy towards them, while Gongsun Zan preferred to use armed force to keep them under control. In 193, Liu Yu rallied an army of about 100,000 and prepared to attack Gongsun Zan. He instructed his troops to focus on eliminating Gongsun Zan and keep casualties and damage as low as possible. However, one of his subordinates, Gongsun Ji (公孫紀), betrayed him and told Gongsun Zan about his plan. Due to Liu Yu's approach of minimising casualties and damage, he lost the battle against the more aggressive Gongsun Zan and was forced to retreat to Juyong County (居庸縣). Gongsun Zan attacked Juyong County, conquered it within three days, captured Liu Yu and brought him as a prisoner to Ji County (薊縣; in the vicinity of present-day Tianjin and Beijing).