During the reign of Emperor Cheng of the Eastern Jin Dynasty, Yu Liang, the elder brother of Emperor Ming's empress, served as the Imperial Secretary and held the reins of government. At that time, the western borders of the Eastern Jin were unstable, so he recommended the minister Wen Qiao to serve as the governor of Jiangzhou (present-day Jiujiang, Jiangxi) to strengthen defenses.
Shortly after Wen Qiao took office, Yu Liang received a report that Su Jun, the governor of Liyang, was plotting rebellion. Yu Liang gathered the court ministers to discuss how to handle Su Jun. Many advocated sending troops to suppress him, but Yu Liang, thinking himself clever, proposed instead to trick Su Jun into leaving Liyang by summoning him to the capital, Jiankang (present-day Nanjing), to serve as Minister of Agriculture.
When Su Jun received the imperial summons, he knew his rebellion plot had been exposed, so he launched an outright revolt and marched his forces toward Jiankang.
When Wen Jiao received news of Su Jun's rebellion, he immediately rallied his troops, preparing horses and sharpening weapons, planning to sail into the capital Jiankang to defend it, while also sending a letter to Yu Liang requesting permission to lead his army into the city.
However, Yu Liang underestimated the rebel Su Jun's strength, believing the capital's forces were sufficient to repel the attack. He replied to Wen Qiao, "My concern for the enemy on our western border outweighs my worry about the rebels at Liyang. You must stay where you are and not cross the Thunder Pool (southeast of present-day Wangjiang County, Anhui) by even a single step, lest you give the western enemy an opening."
Wen Jiao received the order and had no choice but to cancel his plan to lead troops into the capital.
Soon, Su Jun led his rebel army to Jiankang. Yu Liang commanded his troops to meet them in battle, but was defeated, and Jiankang fell.
Yu Liang retreated with his troops to Jiangzhou
Wen Jiao ordered Yu Liang to lead troops in defending the White Stone Camp, while he himself intensified military drills, preparing for a decisive battle against the rebel forces.
Within days, Su Jun's forces arrived at Baishi and launched a fierce assault on the camp. Wen Qiao and Yu Liang led a counterattack, quickly routing the rebels. Pressing their advantage, they pursued and killed Su Jun, crushing the rebellion and recapturing the capital, Jiankang.
The idiom "dare not cross the thunder pool" originated from Yu Liang's letter to Wen Qiao, originally saying "do not cross the thunder pool," meaning Wen Qiao should guard Jiangzhou and not cross the thunder pool to the capital. Later, it evolved into "dare not cross the thunder pool," used to describe someone who dares not exceed certain boundaries or limits.
Source: *Book of Jin*, "Biography of Yu Liang"
Meaning of the Idiom: Later, the Chinese idiom "不敢越雷池一步" came to describe how someone dares not exceed certain boundaries or limits.