According to the "Book of Jin and the Chronicle of Emperor Xuan", Cao Cao once found that Sima Yi had the "appearance of a wolf", looking at him was a person with a bad heart, and he was quite ambitious, but Cao Cao dreamed of three horses eating the same trough, so he was quite jealous of him, and told his son Cao Pi that Sima Yi was not willing to live under the people, and it would inevitably become a scourge in the future.
Cao Cao's prediction later became a reality. In 249, Sima Yi launched a coup d'état in Gaopingling, killing the general Cao Shuang and seizing power alone. After that, the Sima family gradually grasped the military and political power of the State of Wei, and eventually usurped the throne successfully, establishing the Jin Dynasty.
When Cao Cao discovered that Sima Yi had ambitions, he did not kill him for many reasons.
First of all, people like Sima Yi cannot be killed casually.
Sima Yi was born in the famous Sima clan of Hanoi, who had been an official for generations, and was also one of the top famous and prestigious families at that time, with great influence. Moreover, Sima Yi's father Sima Fang also recommended Cao Cao when he was serving as Jing Zhaoyin, and he also had the grace of recommending Cao Cao.
For such a person, cao Cao certainly could not kill Sima Yi on the grounds that he did not look good, or that he felt that the other party would rebel, otherwise it was bound to make everyone within Cao Wei endanger themselves. At this time, Cao Cao still had two fierce enemies in the south, Liu Bei and Sun Quan, and it would not be easy to destroy internal stability.
Secondly, Sima Yi is also very good at pretending.
After Sima Yi found out that Cao Cao was suspicious of himself, he did not show too many abnormalities, nor did he express any complaints, but instead did things more diligently, carefully and conscientiously, low-key people, and showed a harmless appearance of people and animals, so that Cao Cao could not find any excuses, and second, his vigilance was relaxed.
The third is Cao Pi's maintenance of Sima Yi.
Sima Yi and Cao Pi had a good relationship and served under him for a long time. Therefore, in the later dispute between Cao Pi and Cao Zhili, Sima Yi naturally sided with Cao Pi and advised him. Most of these schemes worked well, which made Cao Pi have great trust in Sima Yi.
Therefore, after Cao Cao became suspicious of Sima Yi, Cao Pi repeatedly said good things about Sima Yi and defended them, which undoubtedly eased the relationship between Cao Cao and Sima Yi.
Fourth, and most importantly, Cao Cao did not think that Sima Yi would cause much harm at this time.
Cao Cao was still a bit fierce in his work, and if Sima Yi really had a potential threat, then he would not be soft. However, during Cao Cao's lifetime, including before his death, the rule of the Cao clan in the north was becoming more and more stable, and military power was also in the hands of Xiahou Huan, Cao Ren, Cao Hong, Cao Xiu and other relatives. In contrast, Sima Yi's official position was not too high, and he had no military power, and no matter how ambitious he was, he could not make too much waves.
At the same time, since Cao Cao chose Cao Pi as his heir, he was still more confident in his ability. Even if Cao Pi reused Sima Yi after he succeeded to the throne, Sima Yi's power would increase, and Cao Cao was not worried about what tricks cao Pi could play while he was alive.
As for whether Sima Yi would rebel after Cao Pi's death, considering that Sima Yi was born in 179 AD, Cao Pi was born in 187 AD, and Sima Yi was eight years older than Cao Pi. Normally, he would also die before Cao Pi, which was obviously not a problem.
Cao Cao's idea is correct to a certain extent, in fact, do not say that when Cao Pi was alive, that is, Cao Pi died, and after Cao Rui succeeded to the throne, Sima Yi has not been abnormal.
But what Cao Cao did not expect was that Sima Yi actually lived so long, and Cao Pi and Cao Rui both died young before they were forty years old. As a result, Sima Yi not only killed Cao Pi, but also killed Cao Rui.
To make matters worse, after Cao Pi succeeded to the throne, due to the lingering fear of the original dispute over the establishment of the heir, Cao Pi strictly restricted the participation of the imperial family in government affairs during his reign, resulting in the decline of the clan power. On the other hand, with the promotion of the Nine Pins Zhongzheng system by Cao Pi, the power of the warrior clan in the court was further strengthened, and the balance of strength above the court was slowly unbalanced.
Moreover, Xiahou Huan, Cao Ren, Cao Hong, Cao Xiu, Cao Zhen and others also died one after another. When Cao Rui was dying, he could only rely on Sima Yi and Cao Shuang. At this time, Sima Yi, as the elder of the Three Dynasties and the Two Dynasties, was the most senior minister in the court, and his influence was extremely great, and it was no longer easy to control.
Cao Shuang's ability is limited, and he loves dictatorship, and he presses Sima Yi step by step. In this case, Sima Yi had the ability to usurp power, and was forced into a dead corner, so naturally he would not sit still, and finally launched a coup d'état.
Therefore, Sima Yi finally launched a coup d'état to control cao Wei's military and political power, which was the result of the combined action of many accidental factors. After all, Cao Cao was just an ordinary person, and later these changes had exceeded the possibility he could have predicted, and naturally he would not kill Sima Yi, who was still the right-hand man of his son Cao Pi, who was not yet a substantial threat.