草木皆兵 — “Grass and trees are all soldiers”

During the Eastern Jin Dynasty, Fu Jian, the King of Qin, harbored ambitions to conquer the Jin dynasty. He personally led a massive army of 900,000 soldiers to attack Jin, while Jin's generals Xie Shi and Xie Xuan commanded only 80,000 troops.

Fu Jian was arrogant and initially underestimated the Jin forces. However, in their initial engagement, Qin's vanguard suffered a humiliating defeat, throwing Fu Jian into a panic. That night, he and his brother Fu Rong surveyed the front lines. Fu Jian saw the Jin army's orderly formation and high morale. When he looked north at Bagong Mountain, where the Jin forces were stationed, the shadows of the grass and trees appeared to him like countless enemy soldiers, a manifestation of his growing fear and suspicion.

Soon after, in the decisive Battle of Feishui, the Qin army was utterly crushed, suffering heavy losses. Fu Jian himself was wounded, and his brother Fu Rong was killed. Fu Jian fled in disarray, so terrified that even the sound of wind and birds made him imagine they were pursuing enemy soldiers.

This idiom vividly describes a person consumed by extreme fear, seeing imaginary threats everywhere.