This episode is from the Romance of the Three Kingdoms. Cao Cao sent a letter demanding Zhou Yu's surrender, which Zhou Yu tore to pieces. Jiang Gan, Zhou Yu's former classmate and now an adviser to Cao Cao, volunteered to travel to the Wu camp to persuade Zhou Yu to surrender.
When Zhou Yu learned of Jiang Gan's arrival, he immediately guessed his purpose. Zhou Yu devised a clever plan to thwart Jiang Gan and enlisted a subordinate's help. He greeted Jiang Gan warmly, took him by the hand, and led him to a banquet. Zhou Yu announced to his assembled officers, "This is an old friend. Though he comes from Cao Cao's camp, he is not here to persuade me to surrender. Please do not suspect him."
After the feast, Zhou Yu pretended to be drunk and invited Jiang Gan to his bedroom to rest. Feigning sleep, Zhou Yu gave Jiang Gan the opportunity to search through his documents. Jiang Gan was shocked to find a secret letter from two of Cao Cao's generals, Cai Mao and Zhang Yun, pledging to defect to Zhou Yu. He hid the letter in his pocket and returned to bed.
Later that night, a messenger secretly entered the tent, woke Zhou Yu, and reported, "A messenger from Cao Cao's camp says Cai Mao and Zhang Yun are not yet ready to act." After hearing this, Zhou Yu went back to sleep. Jiang Gan, having overheard the conversation, silently slipped out of the camp, crossed the river, and delivered the letter to Cao Cao. Enraged, Cao Cao had the two generals executed.
When the news reached Wu, Zhou Yu laughed heartily and said, "Cai Mao and Zhang Yun were the two generals I feared most. Now I fear no one." Cao Cao soon realized he had been tricked by Zhou Yu's stratagem of sowing distrust within his own ranks.