This is a famous part of Battle of Red Cliffs's story. Cao Cao's army went down the river to Wu Dong, and Sun Quan ordered Zhou Yu to lead the water army to fight. Cao Cao's youth and Xu did not learn how to fight in the water, but Cai Mao, a general in Jingzhou, was "impressed by the water army" and was appointed commander-in-chief of the water army by Cao Cao. Zhou Yu is determined to use this double agent to get rid of Cai Hezhang. Jiang Gan under Cao Cao was blind and confident, thinking that Zhou Yu could be persuaded to surrender to Cao Cao by friendship. The result was delivered to the door and served as Zhou Yu's "helper" in practicing double-faced espionage.
If you know it carefully, you will find that it is not Jiang Gan who really fell for it, but Cao Cao.