After the An Lushan Rebellion in the Tang Dynasty, the central government's power steadily weakened, and regional military governors began hoarding troops and asserting their own authority, forming local separatist forces.
When Emperor Xianzong ascended the Tang throne, he saw that regional warlords ignored central commands and posed a grave threat, so he resolved to gradually eliminate or weaken their power to strengthen imperial authority.
At the time, Wu Yuanji, the military governor of Huaixi, commanded considerable power and repeatedly defied central authority, so Emperor Xianzong of the Tang Dynasty resolved to send troops against him, yet several campaigns failed to achieve results.
In 817 AD, Emperor Xianzong of the Tang Dynasty decided to launch another campaign against the rebel Wu Yuanji, dispatching the bold and strategic general Li Su to lead the assault on Wu Yuanji's stronghold, Caizhou.
When General Li Su led his army into Caizhou territory, he first devised a plan to capture Wu Yuanji's cavalry commander Li You alive and persuaded him to defect to the imperial court. Then, through Li You and another surrendered general Li Zhongyi, Li Su learned the full deployment of Wu Yuanji's forces.
Li You advised Li Su, "Wu Yuanji's elite troops are all stationed at Huiqu, while only the old and weak guard Caizhou. We can strike while they're unprepared—race a crack force to Caizhou at breakneck speed and attack by surprise. By the time his main army hears the news, the city will already be taken and Wu Yuanji captured alive."
Following Li You's advice, General Li Su led his elite troops on a snowy night to launch a surprise attack on Caizhou, capturing the rebel Wu Yuanji alive and securing a brilliant victory.
Later, the idiom "taking advantage of a weakness" came to describe suddenly striking when the opponent's defenses are empty.
Source: *Zizhi Tongjian*, "Tang Records"
Meaning of the Idiom: Later, the Chinese idiom "乘虚而入" came to describe how suddenly striking when the opponent's defenses are empty.