有一次，魏國的曹操水軍敗給吳國的周瑜後，便找來善水戰的大將蔡瑁及張允來訓練水軍，周瑜知道後，想把這兩位大將除掉，曹操得知周瑜在探查水軍操練情形，心裡很不安。這時，曹軍中最能言善道的蔣幹，表示過去和周瑜是舊識，加上自己的好口才，一定能說服周瑜投效曹操。
當蔣幹到達吳軍軍營時，周瑜便開口對他說：「孫權很重用我，我是不會去投效曹操的。好久不見了，我們今天只話家常不談政治，我準備好酒好菜，咱們不醉不歸。」周瑜心情很好，酒一杯接著一杯喝，最後喝醉倒在一旁睡覺，蔣幹看到書桌上有些公文書信，便想找找看有沒有什麼好情報，竟然看到蔡瑁及張允兩人與周瑜的密函。
隔天一早，馬上回去告訴曹操，兩名大將原來是吳軍的奸細，曹操一時衝動，把兩人立刻斬首，等到事後冷靜思考，才知道中了周瑜的「反間計」，但是為時已晚。
三十六計之第三十三計：反間計
收買敵人的部屬，替自己蒐集或竊取敵方情報，甚至代傳假情報給敵方，使敵人做出錯誤的判斷或決定。
Cao Cao's navy was defeated by Jhou Yu, a general of Wu. So Cao Cao recruited generals Cai Mao and Jhang Yun, who are experts in naval battles, to train an rebuild his navy. When the news reached Jhou Yu, he thought that both generals must be removed in order to prevent future threat. When Cao Cao learned that Jhou Yu was inquiring about his new navy, he was very nervous. At this point, Jiang Gan, a subordinate famous for his eloquence, told Cao Cao that given the fact that he and Jhou Yu were old acquaintances, he is confident that he could persuade Jhou Yu to defect to Cao Cao.
When Jiang Gan arrived at the Wu camp, Jhou Yu said, "The king of Wu regards me highly. I will not offer my services to Cao Cao. In any case, we have not seen each other for a long time. Let's not talk politics tonight but just chat. I have prepared food and wine for you. Let's drink till we drop." Jhou Yu was in an extremely good mood and drank cup after cup until he passed out. Jiang Gan saw some papers lying on Jhou Yu's desk and thought about rummaging through them to look for some useful intelligence. To his shock, he found some secret correspondence between Jhou Yu and the two generals Cai Mao and Jhang Yun.
The next morning, upon his return, he reported what he had learned to Cao Cao immediately. Of course, he told Cao Cao of discovery that Cai Mao and Jhang Yun were actually spies sent by Wu. Cao Cao was furious. On impulse, and without verifying the truthfulness of this information, he executed the two immediately. It was only afterwards, when he reflected on the whole event, that he realized he had been tricked by Jhou Yu's counter-intelligence and disinformation efforts. But it was too late.
Strategy 33: The Art of Counter-Espionage
Bribe your enemy's subordinates to spy on or spread disinformation to your enemy, thereby tricking your enemy into making the wrong judgment or the wrong decision.
