Fish in troubled waters

Explaination: Literally speaking, this idiom means fish would be confused and disoriented in troubled waters, in this case people could take the opportunity to catch it and thus acquire unexpected profits. When used for military affairs, it means the strategy to win victory seizing the opportunity that enemy is in disorder and confusion. In troubled waters, fish could not distinguish directions. In complicated wars, the weaker side often blows hot and cold, thus opportunity exists. More often, this opportunity can not arrive just by waiting, but by making it on one's own initiative. One side actively makes water feculent to make the situation roily , then things could happen.

In 632 BC the armies of Jin and Chu faced each other at Chengbu before the battle of the same name. Chu sent an envoy to Jin requesting to fight a chariot duel the next day to which the Jin ruler, Duke Wen, agreed. In the morning Duke Wen ordered his troops to cut down trees to be used as part of an unorthodox tactic. While the chariot duel was underway Duke Wen launched a sudden cavalry attack against the Chu right wing, and then the Jin troops in the center raised the retreat pennants and began pulling back. As the Jin troops retreated they dragged behind them the trees they had cut down earlier that morning. This raised such a dust cloud that the Chu commanders thought the Jin were fleeing in panic and eagerly gave chase. When the main body of the Chu army was enveloped in the cloud of dust they were unable to see that the Jin forces had split into two divisions and had turned around. The result was a resounding defeat after which the Chu general was ordered to commit suicide.