[Explanation]: To release a tiger back into the mountains. This idiom metaphorically means releasing a dangerous person back to their stronghold, leaving behind a hidden danger.
[Story]: After personally leading his army to defeat Lü Bu, Liu Bei followed Cao Cao to the capital city of Xuchang. Although Cao Cao outwardly treated Liu Bei with great respect, he was actually very suspicious and often sent people to spy on Liu Bei's activities. At that time, Emperor Xian of Han, who had been forced by Cao Cao to move to Xuchang, was secretly issuing imperial edicts to organize a group of people to assassinate Cao Cao. As a member of the Han imperial family, Liu Bei also participated in this secret plot. To avoid arousing Cao Cao's suspicion, he often closed his gates and stayed in his courtyard growing vegetables, pretending to have no grand ambitions.
The strategist Cheng Yu (Note: "Cheng Wu" in the text is likely a typo for Cheng Yu) realized that Liu Bei was no ordinary man. He said to Cao Cao: "I see that Liu Bei has great ambitions and possesses a heroic spirit. If we do not kill him now, he will surely become a disaster in the future." Uncertain, Cao Cao sought the opinion of another strategist, Guo Jia. Guo Jia believed that, at this time of needing talented people, Liu Bei was a hero who had come to join Cao Cao only after defeat. Killing him would earn Cao Cao a bad reputation for harming the worthy, bringing no benefit. Cao Cao agreed with Guo Jia's reasoning.
Soon after, Yuan Shu, having been defeated by Cao Cao's forces, planned to flee and join Yuan Shao. Unwilling to allow the two powerful forces of Yuan Shu and Yuan Shao to unite, Cao Cao prepared to send troops to intercept Yuan Shu. Seeing his chance to escape, Liu Bei said to Cao Cao: "Yuan Shu must pass through Xuzhou to reach Yuan Shao. Please grant me some troops to intercept him en route; I guarantee we will capture Yuan Shu." Cao Cao, suspecting nothing, reported to Emperor Xian and sent Liu Bei to lead 50,000 troops to Xuzhou. Liu Bei immediately set off in haste. Guan Yu and Zhang Fei, seeing their brother's frantic state, were puzzled and asked: "Brother, why are you in such a hurry for this campaign?" Liu Bei explained: "Under Cao Cao, I am like a bird in a cage or a fish in a net—neither safe nor able to use my talents. This campaign is like a fish returning to the ocean or a bird flying into the sky. I can now swim and soar freely, no longer restrained by anyone."
Immediately after Liu Bei departed, Guo Jia and Cheng Yu returned to Xuchang from elsewhere. Upon hearing that Cao Cao had let Liu Bei go, they rushed to see him. Cheng Yu said: "When Liu Bei was Governor of Yuzhou, we once requested you to execute him, but the Chancellor did not listen. Now you have given him a large army—this is like releasing a dragon back into the sea or returning a fierce tiger to the mountains! How will we ever be able to subdue him again in the future?" Guo Jia added: "Even if the Chancellor chooses not to kill him, he should not have let him leave so easily. As the saying goes, once you let an enemy escape, it will bring endless trouble."