In the final years of the Tang Dynasty, Huang Chao, a salt merchant from Caozhou, led a massive rebellion. After his army captured Guangzhou in the south, they marched north and seized the Tang capital, Chang'an, forcing Emperor Xizong to flee in panic.
Emperor Xizong of Tang, to stop the rebel army's advance, appointed Zheng Tian, a jinshi degree holder, as the military governor of Fengxiang. With tears in his eyes, he said to Zheng Tian, "Fengxiang is a strategic location—you must hold it firmly for me!"
Zheng Tian said to Emperor Xizong, "Fengxiang is far from Your Majesty, and the war situation changes constantly. It would be very inconvenient to seek your instructions during battle. Could you grant me the authority to act as I see fit?" The emperor replied, "As long as it benefits the court, fight this war however you see fit."
Upon arriving in Fengxiang, Zheng Tian repaired war chariots, expanded the army, reinforced the city walls, and distributed his personal wealth to the soldiers. Day and night, the troops remained on constant alert, as if facing an enemy at any moment.
Soon after, Huang Chao's general Shang Rang led fifty thousand troops to attack Fengxiang. Believing his forces were overwhelming and that Zheng Tian, being a scholar, knew nothing of military strategy, Shang Rang marched his army in disarray, advancing recklessly with contempt for the enemy. He fell into Zheng Tian's ambush, suffered a crushing defeat, and fled back with over twenty thousand soldiers lost.
Later, the idiom "as if facing a formidable enemy" came to describe taking a situation extremely seriously.
Source: *Old Book of Tang*, "Biography of Zheng Tian"
Meaning of the Idiom: Later, the Chinese idiom "如临大敌" came to describe how taking a situation extremely seriously.