During the Sixteen Kingdoms period, Fu Jian of Former Qin ascended the throne and gradually unified most of northern China. In 383 AD, he assembled an army of 900,000 troops, intending to conquer the Eastern Jin and unify the entire realm. Facing the Qin army's southward advance, the Eastern Jin court appointed Xie Shi as the general and Xie Xuan as the vanguard, leading 80,000 elite soldiers to block the Qin forces.
The vanguard of the Qin army, led by Fu Rong, swiftly captured Shouyang (present-day Shou County, Anhui Province). Upon hearing the news, Fu Jian personally led 8,000 cavalry to Shouyang. Trusting Fu Rong's assessment that the Jin forces were weak and no match for them, Fu Jian believed they would panic upon learning of the Qin army's arrival. So he sent a man named Zhu Xu to persuade Jin commander Xie Shi to surrender.
Zhu Xu, originally an Eastern Jin official, defected to the Jin camp and reported the Former Qin army's deployment to Xie Shi. Accepting his advice, Xie Shi dispatched 5,000 cavalry to secretly cross the Fei River and launch a night raid on Luo Stream (modern-day East Luo River in Huainan, Anhui). The caught-off-guard Qin forces suffered 15,000 casualties. The Jin army then counterattacked toward Shouyang, spreading their troops along the eastern bank of the Fei River.
When Fu Jian learned of the defeat at Luo Stream and that Jin forces were counterattacking toward Shouyang, he was startled. Dragging his general Fu Rong and other commanders, he climbed the walls of Shouyang to personally observe the Jin army's movements on the eastern bank of the Fei River.
It was the dead of winter, and overcast skies hung low over the Fei River, a dreary expanse of gray. Yet on the eastern bank, masts bristled like a forest, warships crowded the shore, and Jin soldiers with swords and halberds patrolled the dikes. In the distance, banners fluttered amid the shouts and neighs of countless Jin cavalry drilling.
After a careful inspection, Fu Jian couldn't help but secretly admire the Jin army's orderly defenses and well-trained troops. After a moment, he turned his gaze toward the Bagong Mountain to the north. He might have been better off not looking—one glance and he was frozen in terror.
The Bagong Mountains stood north of the Huai River, west of the Fei River, just four li from Shouyang City, with eight rugged peaks that made the terrain extremely treacherous. The Jin army's main camp was stationed at the foot of these mountains. As Fu Jian surveyed the range, a northwest wind swept through, causing him to vaguely notice the swaying grass and trees on the peaks—they looked like countless soldiers charging. His face instantly turned ashen, and he turned to Fu Rong, saying, "This is a formidable enemy! How can you call them weak?"
"The Jin army is a formidable opponent—how can you call it weak?"
Despite his inner fear, he clung to the belief that his overwhelming numbers could crush the Jin army. To play it safe, he ordered his forces to gather by the Fei River east of Shouyang, waiting for reinforcements before launching the attack.
A few days later, Xie Xuan devised a clever plan, sending an envoy to demand that Fu Jian retreat slightly to allow the Jin army to cross the Fei River for a decisive battle. By then, Fu Jian had forgotten the terror of seeing every bush and tree on Bagong Mountain as enemy soldiers, and he fell for the trick, agreeing to the retreat. As a result, the Qin army trampled each other in chaos during the retreat, collapsing into a disorganized rout, with soldiers suffering from cold and hunger along the way, and seven or eight out of every ten perished.
Later, people derived the idiom "Every bush and tree looks like an enemy" from this story, describing someone who is so terrified that they become paranoid and nervous at the slightest movement.
Source: *Book of Jin*, "Biography of Fu Jian, Part Two"
Meaning of the Idiom: Later, the Chinese idiom "草木皆兵" came to describe how someone is so terrified that they become paranoid and nervous at the slightest movement.