Double Agent Scheme, 反间计 - 36 Strategies
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36 Strategies - Strategy No 33: Double Agent Scheme, 反间计
Original: 疑中之疑。比之自内，不自失也。
Modern: 在敌人给我布置的疑阵中再反设一层疑阵。如果利用敌人内部的策应去争取胜利，那么我方就不会遭受损失。
English: When an enemy lays traps, set trap within his traps to create internal chaos. When one secures assistance within enemy's ranks, one will not lose the battle.
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Illustration
During the Three Kingdoms Period, one of the great illustration of the Double Agent Scheme is seen in Battle of the Red Cliff. Cao Cao initially had Cai Mao and Zhang Yun as his chief marine commanders for his 830,000 troops. As Cao Cao's troops came from the northern side, his soldiers does not have any experience in marine battles, so Cai Mao and Zhang Yun was extremely precious to Cao Cao.
On the other side of the battle is the alliance of Liu Bei and Sun Quan with Zhou Yu as the commander-in-chief. Zhou Yu was very talented and intelligent, thus Cao Cao had the intention of pulling Zhou Yu to his side. Zhou Yu knowing that Cai Mao and Zhang Yun was the chief marine commander was not happy, he was thinking of a plan to get rid of them when he heard Jiang Gan was coming.
Jiang Gan served Cao Cao and was a former classmate of Zhou Yu. Zhou Yu knew that Jiang Gan's intention was to persuade him to switch side, but he intend to use Jiang Gan to remove Cai Mao and Zhang Yun. Zhou Yu threw a feast for Jiang Gan and ordered that during the feast, military affairs should not be mentioned. The feast went into the middle of the night. After the feast, Zhou Yu pretended to be very drunk and invited Jiang Gan to spend the night in his tent. Jiang Gan on seeing that he has not achieved his goal was not able to sleep so he wandered around in Zhou Yu's tent. He stumbled upon a letter that was written by Cai Mao and Zhang Yun, stating that they are willing to work with Zhou Yu to defeat Cao Cao.
Suddenly, Zhou Yu talked in his dreams. It scared Jiang Gan. He kept the letter in his sleeves and went back to his bed. After a while, someone came to wake up Zhou Yu. Zhou Yu woke up and spoke to the messenger while keeping his eyes on Jiang Gan. Jiang Gan strained his ears to listen to the conversation but he can only vaguely hear the words "Cai" and "Zhang". Jiang Gan then confirmed his suspicions.
Jiang Gan hurried back to Cao Cao's camp after he made sure that Zhou Yu has fallen asleep. He showed the 'fake' letter to Cao Cao and told Cao Cao about the conversation. These infuriated Cao Cao and he ordered the execution of Cai Mao and Zhang Yun. After a short while, Cao Cao realize his mistake but it was too late, he has lost his two precious marine commanders.
三国时期，赤壁大战前夕，周瑜巧用反间计杀了精通水战的叛将蔡瑁、张允，就是个有名例子。
曹操率领号称的八十三万大军，准备渡过长江，占据南方。当时，孙刘联合抗曹，但兵力比曹军要少得多。
曹操的队伍都由北方骑兵组成，善于马战，可不善于水战。正好有两个精通水战的降将蔡瑁、张允可以为曹操训练水军。曹操把这两个人当作宝贝，优待有加。一次东吴主帅周瑜见对岸曹军在水中排阵，井井有条，十分在行，心中大惊。他想一定要除掉这两个心腹大患。
曹操一贯爱才，他知道周瑜年轻有为，是个军事奇才，很想拉拢他。曹营谋士蒋干自称与周瑜曾是同窗好友，愿意过江劝降。曹操当即让蒋干过江说服周瑜。
周瑜见蒋干过江，一个反间计就已经酝酿成熟了。他热情款待蒋千，酒席筵上，周瑜让众将作陪，炫耀武力，并规定只叙友情，不谈军事，堵住了蒋干的嘴巴。
周瑜佯装大醉，约蒋干周床共眠。蒋干见周瑜不让他提及劝降之事，心中不安，哪里能够入睡。他偷偷下床，见周瑜案上有一封信。他偷看了信，原来是蔡瑁、张允写来，约定与周瑜里应外合，击败曹操。这时，周瑜说着梦话，翻了翻身子，吓得蒋干连忙上床。过了一会儿，忽然有人要见周瑜，周瑜起身和来人谈话，还装作故意看看蒋干是否睡熟。蒋干装作沉睡的样子，只听周瑜他们小声谈话，听不清楚，只听见提到蔡、张二人。于是蒋干对蔡、张二人和周瑜里应外合的计划确认无疑。
他连夜赶回曹营，让曹操看了周瑜伪造的信件，曹操顿时火起，杀了蔡瑁、张允。等曹操冷静下来，才知中了周瑜反间之计，但也无可奈何了。