In 207 AD, Cao Cao led a massive army to attack Liu Bei, who was forced to retreat with a cumbersome column of thousands of carts and a mixed crowd, managing only a dozen li per day. Cao Cao handpicked his elite troops and raced at breakneck speed, covering three hundred li in a single day and night, finally catching up with Liu Bei at Changban in Dangyang. Liu Bei, along with Zhuge Liang, Zhang Fei, Zhao Yun, and a few dozen riders, barely escaped, while the rest of his people and supplies were all captured by Cao's forces.
During the crisis at Dangyang, Liu Bei received Lu Su, an Eastern Wu general sent to offer condolences for Liu Biao's death. Lu Su asked about Liu Bei's plans, and Liu Bei replied he intended to seek refuge with Wu Ju, the Grand Administrator of Cangwu. Lu Su advised, "Wu Ju is a mediocre man in a remote place—what future could you find there? But Sun Quan is wise, benevolent, and honors talent. He already holds six commanderies east of the Yangtze, with strong troops and ample supplies—a true leader for great deeds. Your best course now is to send an envoy to form an alliance with Eastern Wu against Cao Cao."
Liu Bei was delighted. After deliberation, he moved his forces to Xiakou and immediately dispatched Zhuge Liang to meet Sun Quan in the Eastern Wu, forging the Sun-Liu alliance. Then, the two sides joined forces and fought together, crushing Cao Cao's army of hundreds of thousands in the historic Battle of Red Cliffs.
After the battle was won, Liu Bei personally traveled to the Eastern Wu capital to meet Sun Quan.
At this moment, Zhou Yu, the great general of Eastern Wu, submitted a memorial to Sun Quan, saying, "Liu Bei is a formidable hero of his time, and with fierce generals like Guan Yu and Zhang Fei to assist him, he will never willingly remain under another's command. I believe the best course is to move him to Wu City, build him a splendid palace, and provide him with beautiful women and luxuries to indulge his pleasures, thereby dulling his ambition and separating him from Guan Yu and Zhang Fei. Once this is done, I can lead troops to attack them one by one, and the problem will be resolved. If we do not act, once he gains a foothold and the three of them unite, it will be like a dragon obtaining rain and clouds—he will no longer be a creature of the pond."
With the formidable enemy Cao Cao eyeing the Jiangnan region, Sun Quan needed to leverage Liu Bei's forces to resist together, yet he also feared that Liu Bei and his allies would not be easily subdued, so Sun Quan ultimately rejected Zhou Yu's proposal.
Things turned out exactly as Zhou Yu had predicted. Later, relying on the Jing-Xiang region "borrowed" from Eastern Wu, Liu Bei took Hanzhong in the west and eventually formed the Three Kingdoms with Cao Cao and Sun Quan. "Not a creature of the pond" means not an animal confined to a pond. It is later used to describe someone with great ambition.
Source: *Records of the Three Kingdoms*, "Book of Wu: Biography of Zhou Yu"
Meaning of the Idiom: Later, the Chinese idiom "非池中物" came to describe how someone with great ambition.