In the Eastern Jin Dynasty (317-420), the king of Former Qin Fu Jian led an army to attack Jin State south of the Yangtze River. The Jin generals Xie Shi and Xie Xuan mustered 80,000 soldiers to counterattack. Fu Jian wanted to take the chance to defeat the Jin army rapidly, since it was much smaller in number.
In October of the same year, the Qin army occupied Shouyang Town and exploited the victory to besiege Xie Shi. Fu Jian led 8,000 hand-picked troops to Shouyang to spearhead the campaign. However, Xie Xuan took the opportunity to send General Liu Laozhi to assault Luojian with 5,000 troops,killing 10,000 Qin soldiers. The Qin army was badly beaten and its morale severely shaken; the soldiers were utterly demoralised.
Fu Jian himself had not expected that the Jin army had such mighty battle effectiveness. He quickly climbed up the city wall of Shouyang to observe the Jin army, only to witness a formidable battle array aligned against him. And then, looking at the Bagongshan Mountain in the north, he mistook the grass and trees there for enemy soldiers, saying, “What a strong army! How can anyone say that it is lacking in numbers?” He began to regret having underestimated the enemy.
Subsequently, he established his troops on the northern bank of the Feishui River. At that time, Xie Xuan asked whether the Qin army could back off a little so as to allow the Jin army to cross the river and do battle. Fu Jian snickered that the Jin generals were ignorant of the basic principles of military tactics and intended to make a sudden attack on the Jin troops when they were busy crossing the river, and so he jumped at the offer made by the Jin army. However, immediately after the order of recession was issued, the Qin troops backed off like tidewater and were comprehensively routed because the troops behind were not aware of why they were backing off and assumed the forward troops had been defeated. The Qin troops were even trampled to death in the stampede. The Jin troops, meanwhile, took the opportunity to cross the river and persue the enemy, killing large numbers. Fu Jian was hit by an arrow but escaped to Luoyang. On the way, he shook with fear at the mere rustle of leaves in the wind. Soon, the former Qin Dynasty collapsed.
The story is from HistoryoftheJin Dynasty:BiographyofFuJian. It is used to describe the state of extreme nervousness or alarm in one’s own group.