Confucius had two disciples, Ran You and Ji Lu, who both served as stewards in the household of the Ji brothers, the three powerful lords who had seized control of Lu’s politics, economy, and military, reducing the duke to a mere puppet. When the Ji brothers plotted to annex the small neighboring state of Zhuanyu, Ran You and Ji Lu felt uneasy and went to sound out Confucius’s thoughts.
Confucius criticized them, saying, "What you have done is wrong. Zhuanyu was also enfeoffed by the former kings to govern the eastern tribes. It has committed no fault now; it has merely declined. How can you send troops to attack it?"
Ran You said, "If the Ji family insists on doing this, there's nothing we can do about it!"
Confucius said, "A weak neighbor like Zhuanyu is in dire straits—you should lend a helping hand. Instead, you not only fail to help but also plan to attack it. What good are you then? Besides, letting the tiger out of its cage and destroying the jewels in the casket—isn't the person responsible at fault?"
When Ran You argued, "Zhuanyu borders Lu's territory; now is the perfect time to seize it while it's weak, or it may trouble our descendants," Confucius replied, "In my view, the Ji family's greatest worry is not that border state of Zhuanyu, but within their own walls—among the Ji brothers themselves."
Soon after, the Ji brothers indeed fell into internal strife among themselves.
Later, people used the idiom "Trouble Rises Within the Walls" to describe internal strife.
Source: *The Analects*, Chapter "Ji Shi"
Meaning of the Idiom: Later, the Chinese idiom "祸起萧墙" came to describe internal strife.