Sima Zhao had two sons that he was proud of, the eldest son Sima Yan and the second son Sima You. These two sons were very good, each with its own strengths, so Sima Zhao couldn't decide before he died, which one to let inherit his career?
Sima You was Sima Zhao's second son, young and intelligent, mild-mannered, well-read, and capable of governance, and because Sima Shi had no children, he was succeeded to Sima Shi.
In his later years, Sima Zhao wanted to make Sima You crown prince, and said that the throne was originally the son of his brother Sima Shi and should be passed to Sima Shi's "son" Sima You. However, some ministers opposed it, and the minister said that Sima Yan had an imperial atmosphere, his hands were over his knees, his hair was mopping the floor, he was not the face of a human subject, and Sima Yan should be made crown prince. Sima Zhao had no choice but to pass the throne to Sima Yan.
In 265, Sima Yan succeeded his father Sima Zhao's title as King of Jin, and a few months later forced the Wei Yuan emperor Cao Wanchan to give him the title of Jin, established the capital Luoyang, and changed the name of Yuan Taishi to Yuan Taishi. Sima Yan became the founding prince of the Jin Dynasty.
History proves that passing the throne to Sima Yan may have been the biggest mistake sima Zhao made, and Sima Yan was very unsuccessful in a major decision as emperor, which also caused the "butterfly effect". Those ministers who supported Sima Yan were deceived by Sima Yan's long arms and long hair, and it seems that people really can't look at each other, or do they really deserve "long hair and short knowledge"?
In view of the fact that the Wei emperor Cao Pi wantonly reduced the military power of the prince surnamed Cao, resulting in the fact that after the Gaopingling Rebellion, the Cao clan relatives actually did not have the troops to escort the King of Qin, and after Sima Yan established the Jin Dynasty as emperor, he was worried that if a similar mutiny occurred in this dynasty, the prince surnamed Sima might also be helpless. Therefore, Sima Yan enfeoffed the kings of the same surname, took the county as the state, and asked them to recruit soldiers, hoping that they would restrain each other, and if there was an emergency in the imperial court, they could also defend the imperial court. But Sima Yan did not expect that so many princes who supported soldiers and respected themselves needed a strong emperor to live in the town. This laid an important hidden danger for the "Rebellion of the Eight Kings" that followed.
Afraid of what would come, Sima Yan's successor after his death was his cowardly and incompetent son Sima Zheng, so the Jin Dynasty became a pot of porridge.
In a centralized absolute monarchical society, it is crucial to choose a successor, otherwise it is "his rise and fall, and his death is also sudden." The selection of successors under the hereditary system is like having to choose a better sheep in a specific flock of sheep, which is very difficult, and not everyone can have children like Sun Zhongmou.
But more important than choosing a successor is how to successfully pass the throne to the successor before his death, Qin Shi Huang mishandled things, and let his son Hu Hai steal Fu Su's inheritance rights is an example, resulting in the Qin Dynasty being buried in the hands of Hu Hai. Sima Yan's indecisiveness, whoever succeeded to the throne before his death, was already controlled by ministers and foreign relatives, and finally made his foolish son Sima Zheng emperor.
Sima Yan was a good emperor, and during his reign there was a "Taikang prosperous era". However, no matter how hard he tried to govern and how well he governed the country before he died, if he could not pass on the throne to others, everything would be in vain. Therefore, it is not so much that the Western Jin Dynasty died at Sima Zheng as it was that it died at Sima Yan.