Jia Xu was a remakable tactician. Many sought him out, knowing that he would provide strategy regardless of their loyalties. As an advocate of the late Dong Zhuo, he found he had many enemies. To protect himself, he coaxed Li Jue and Guo Si into assailing the new government in Changan. After settling the capital, Jia Xu won the favor of Emperor Xian, yet frequent quarrels between Li Jue and Guo Si threatened the emperor's safety. Therefore, he created a scheme by which he weakened Li Jue.
     Later Jia Xu became advisor of Zhang Xiu, and helped him establish an alliance with Liu Biao. When Cao Cao tried to destroy the alliance, Zhang Xiu considered surrendering. But then Jia Xu raided their enemy's camp by night. Although Cao Cao escaped, he lost Dian Wei, his oldest son Cao Ang, and a nephew in the assault.
     Cao Cao renewed his attack, but once again Jia Xu forced him to withdraw. Zhang Xiu insisted on chasing after, but was sorely routed by Cao Cao's rear guard. When he returned, Jia Xu urged him to attack the rear again. This time Cao Cao met utter defeat and retreated in haste. Zhang Xiu was confused by Jia Xu's reasoning. "I predicted the plan that Cao Cao would adopt in his retreat," Jia Xu explained. "When he strengthened his rear guard to protect his troops, I knew you would have to tackle his best men. Therefore I opposed pursuit. However, his second retreat was in such haste that I knew his rear guard would not be as prepared this time, and I suggested you pursue him again."
     When Cao Cao and Yuan Shao confronted each other in the Battle of Guandu, Yuan Shao sent a messenger to plead reinforcements from Zhang Xiu. Jia Xu reproached the messenger saying, "How can Yuan Shao beg for help from a distinguished patriot when he refuses to aid a man of his own blood (Yuan Shu)?" From that point, he encouraged Zhang Xiu to join forces with Cao Cao. Jia Xu explained, "Cao Cao pledged his service to the emperor and wields his authority. Yuan Shao commands a mightier army than ours, so our loyalty will mean nothing to him. On the other hand, Cao Cao has a small force, and he will gladly accept our support. Remember that those who aspire for the good of the empire must release their grudges and pursue their duty into the four winds." Zhang Xiu respected Jia Xu's opinion and submitted to Cao Cao. As expected, Cao Cao accepted them, and Jia Xu was assigned honorable tasks.
     As one of Cao Cao's advisors, Jia Xu implemented many clever plans. For instance, when Cao Cao confronted Ma Chao and Han Sui at the River Wei, he devised an alienation maneuver to break them apart. He had Cao Cao send Han Sui a letter filled with obvious revisions. Ma Chao delivered the letter directly into Han Sui's hands. When he saw the state of the letter, Han Sui grew suspicious of Ma Chao, and eventually their friendship was destroyed.
     Jia Xu maintained a long-lasting relationship with Cao Cao. Thus he often had a say in imperial matters. For example, he helped Cao Cao chose an heir amongst his two sons, Pi and Zhi. Cao Pi consulted him for a means to the throne, and he was told, "Let your tears fall freely whenever your father leaves." Thereafter, Cao Cao saw his tears as an expression of filial piety, which Cao Zhi seemed to lack. When Cao Cao asked him which boy to appoint, Jia Xu pondered in silence. Then he explained, "I was just thinking about Yuan Shao and Liu Biao" (both of whom neglected to assign an heir before they died, which eventually led to the downfall of their families). Cao Cao began to laugh when he realized the meaning of Jia Xu's words, and from that moment he named Cao Pi the Crown Prince.