During the Eastern Jin Dynasty, after unifying northern China, Fu Jian grew confident in the strength of his vast army.

In AD 383, he led a force said to number more than 800,000 soldiers southward, determined to conquer the Eastern Jin in a single campaign.

In response, the Eastern Jin sent an elite force of only about 80,000 troops to meet the invasion.

Before the decisive battle, Fu Jian climbed the city walls to observe the Jin army.

Seeing their orderly formations and high morale, he began to feel uneasy.

Looking toward Mount Bagong, he noticed the trees and grass swaying in the wind.

From a distance, he mistook them for enemy soldiers lying in ambush and exclaimed, "Those are formidable troops as well. Who says the Jin army is small?" In reality, there were no hidden soldiers -- only trees and grass.

But Fu Jian's fear and anxiety had clouded his judgment, causing him to imagine enemies where none existed.

Soon afterward, Fu Jian suffered a crushing defeat in the famous Battle of Feishui and was forced to flee in disorder.

The idiom is used to describe someone who is so frightened or nervous that they mistake ordinary things for signs of danger or enemies.