









乘虚而入 is a Chinese idiom (chengyu) meaning “to take advantage of a weak point or opening”. To exploit a vulnerability. Literally, “To enter by taking advantage of a weakness”.
In 817 AD, Emperor Xianzong of the Tang Dynasty appointed Li Su, a general known for his wisdom and bravery, as the Jiedushi (military commissioner) of Tanghe and three other prefectures. His mission was to attack Wu Yuanji, the son of the rebellious Huai-Xi Jiedushi.
Li Su first won over several surrendered generals, including Ding Shiliang, Li You, and Li Zhongyi.
Then, seizing an opportune moment when the enemy was unprepared, he stealthily entered Caizhou, captured Wu Yuanji alive, and quelled the rebellion.