During the Battle of Red Cliffs, Sun Quan and Liu Bei allied against Cao Cao, defeating his claimed million-strong army and preserving Eastern Wu's foundation. After the battle, Sun Quan appointed Liu Bei as "Left General" to govern Jingzhou.
Zhou Yu, deeply uneasy about Liu Bei's growing power, seized the opportunity when Liu Bei came to the capital (present-day Zhenjiang, Jiangsu) to pay respects to Sun Quan. He submitted a memorial to Sun Quan, advising: "Liu Bei is a fierce and ambitious hero, flanked by Guan Yu and Zhang Fei, two bear-like, tiger-like generals. They will never willingly bow to serve you. In my humble opinion, you should relocate Liu Bei to Wu lands, build him grand palaces, lavish him with beauties and treasures to indulge his pleasures. Then separate Guan and Zhang, stationing them under our generals' control to keep them in check. This would secure the realm. But if you grant him territory to expand his cause and let these three remain together, Liu Bei will be like a dragon gaining clouds and rain—he is no mere creature of a pond!"
Sun Quan believed that Cao Cao in the north was eyeing Eastern Wu like a tiger, so he should gather heroes from across the land, and besides, subduing Liu Bei hastily was no easy task; thus, he did not adopt Zhou Yu's advice. As Zhou Yu had predicted, Liu Bei later became the ruler of Shu Han, dividing the realm with Cao Wei and Sun Wu into three kingdoms.
Later, the idiom "a creature in a pond" came to describe someone confined to a corner with no grand ambitions.
Source: *Records of the Three Kingdoms*, "Book of Wu: Biography of Zhou Yu"
Meaning of the Idiom: Later, the Chinese idiom "池中之物" came to describe how someone confined to a corner with no grand ambitions.