When you have finished the battle with cao ren when you start a Liu Bei
game and Lu Bu asks for an alliance, agree to it. After you have agreed,
the game progresses. In the next "chapter" if you look to the north,
Gogsunzan's forces are deafeated by Yuan Shao. If you send Liu Bei as an
envoy to Gogsunzans capital, he and Zhao Yun will join Liu Bei. These
two characters really do make a difference when fighting Lu Bu.