During the Three Kingdoms period, Sima Zhao, son of Sima Yi, inherited his father and brother's legacy and became Grand General and Chancellor of Wei. He ruled with absolute authority, showing no regard for the Wei emperor Cao Mao.
Emperor Cao Mao of Wei, seeing himself reduced to a puppet and Sima Zhao growing increasingly arrogant, was deeply angered. He picked up his brush and wrote a poem titled "Hidden Dragon," comparing Sima Zhao and his ilk to loaches and eels. Upon reading the poem, Sima Zhao flew into a rage and publicly berated Cao Mao in the court:
"We, the Sima clan, have rendered great service to Wei. Why do you compare us to eels and loaches?"
Cao Mao and his ministers trembled in terror, unable to utter a word. Sima Zhao let out a cold laugh and strode away without a backward glance.
Back in the palace, Cao Mao felt that Sima Zhao harbored ambitions to usurp the throne and resolved to strike first to eliminate him. So, he summoned Imperial Advisor Wang Shen, Minister Wang Jing, and Attendant-in-Ordinary Wang Ye to secretly discuss a plan. He said to them indignantly:
"Sima Zhao's intentions are known to all. I cannot sit idle and suffer the humiliation of being deposed—you must help me march against him."
Wang Jing, knowing that the Cao family's power was already lost, tearfully urged Cao Mao to continue enduring. Meanwhile, Wang Shen and Wang Ye, driven by cowardice and self-preservation, prepared to report to Sima Zhao as soon as they left the palace.
Cao Mao made up his mind
Just outside the South Gate, Jia Chong, Sima Zhao's trusted aide, led thousands of guards to intercept him. Cao Mao shouted, "I am the Son of Heaven—do you intend to commit regicide?" The guards, afraid to offend the emperor, began to retreat. Seeing this, Jia Chong cried out, "What did Lord Sima keep you for if not for this very day?"
The guards, jolted by his shout, finally understood and, raising their spears together, killed Cao Mao on the spot.
After Cao Mao's death, Sima Zhao installed another puppet emperor, Cao Huan. Later, Sima Zhao proclaimed himself Duke of Jin, then King of Jin. In 265 AD, Sima Zhao died of illness, and his son Sima Yan succeeded him as King of Jin. A few months later, Sima Yan forced Cao Huan to "abdicate" in his favor, making himself emperor and establishing the Western Jin dynasty, historically known as Emperor Wu of Jin.
Later, people used the idiom "Sima Zhao's Intent" to describe a widely known conspiracy or ambition.
Source: *Records of the Three Kingdoms*, "Biography of the Duke of Gaogui"
Meaning of the Idiom: Later, the Chinese idiom "司马昭之心" came to describe a widely known conspiracy or ambition.