Guo Jia
According to the "Fu Zi", before the Battle of Guandu, Cao Cao wanted to attack Liu Bei (who had rebelled and seized control of Xu Province) first before dealing with Yuan Shao. However, some of Cao Cao's advisers expressed their worries that Yuan Shao would take advantage of their absence to attack them, rendering them trapped between Yuan and Liu Bei. Cao Cao faced a dilemma so he asked Guo Jia for his opinion. Guo Jia suggested, "Yuan Shao is suspicious and he hesitates before he makes any decision, so he won't make his move fast. Liu Bei had just taken control of Xu Province so his situation isn't stable yet. He can be easily defeated if we attack him fast and hard. This is a critical opportunity, we mustn't lose it." Cao Cao agreed and immediately led an army east to attack Liu Bei and defeated the latter. After his defeat, Liu Bei fled to Hebei to join Yuan Shao. Throughout that period of time, Yuan Shao did not make any move.