I’m sure there’s an in-depth article explaining why Jin fell and so forth. Western Jin is interesting for being a brief period of unification for China because from the end of the Han dynasty to the beginning of the Sui dynasty, China was divided.
Since you mention it...
It is very very complicated why the Western Jin collapse.
First, the Western Jin (50 years) actually lasted longer than the esteemed Qin (15 years) and Sui Dynasties (37 years). It also lasted longer than its rivals Cao Wei (46 years), Shu Han (42 years), and was on par with Eastern Wu (51 years).
Second, Sima Yan was a very capable Emperor. The recorded population under Sima Yan (24.8mil+) was actually higher than the recorded population under Latter Han Emperor Guangwu (21mil+) after their respective unifications.
Now, let us start.
The Sima family made a bad start... The founding of the Western Jin Dynasty faced many problems, most of which came from the following aspects
First, the Sima family took too long to usurp Cao Wei. It took a total of 16 years from the time Sima Yi launched the Gaoping Coup and took control of Cao Wei's political situation until Sima Yan replaced Wei and established itself. After that, it took the Western Jin Dynasty another 15 years to unify the world, a total of 31 years between the three generations and four people. The so-called long nights and many dreams mean that the longer the delay, the more complicated the transfer of power will be, and the greater the cost will be. Sima Yi himself was a powerful regent, not a heredity monarch, let alone an emperor. What qualifications did Sima Shi have to inherit his position of power? So after Sima Yi's death, not to mention Taifu, even Sima Shi became the Grand General only after two years. Moreover, there were many people who were dissatisfied with him at home and abroad. Zhang Ji, Li Feng, and Wen Qin Guanqiu Jian all launched actions against Sima Shi. Sima Shi himself also died in the aftermath of putting down a rebellion.
Sima Zhao's governing policy was much more well-rounded than that of Sima Shi, the policy was to vigorously win over powerful nobles and gentries from all walks of life and tie them to the chariot of the Sima family. Sima Shi also used this trick, but the rewards given by Sima Zhao were obviously much heavier. Whenever something big happened in the Sima family, they would reward people like crazy.
Take Jia Chong, this kingslayer, as an example. Before Sima Zhao came to power, Jia Chong was just a tinghou with 400 households. Sima Shi destroyed Wen Qin and Guanqiu Jian and to celebrate, the number of households increased to 750. Sima Zhao destroyed Zhuge Dan and celebrated again, and it became 1750 households. After killing Cao Mao, the number of households increased to 2,950. How many years do you think it took? Five years. Compare the seven to eight hundred households of Jia Xu, Hua Xin and Wang Lang, the 3 Founding Excellencies of Cao Wei, and you will know how exaggerated Sima Zhao's rewards were.
And Sima Zhao couldn't just pamper his own lackeys. He had to get the support of everyone, including the veterans of Cao Wei, his relatives, and the feudal officials. Cao Zhen, the clan relative and leading general who controlled the central army in Cao Wei, only had 2,900 households at his death. During the Sima Zhao period, the generals in border towns had more than 5,000 households per capita.
It's even more exaggerated when it comes to Sima Yan. After all, Sima Zhao fought some battles during his time as did Sima Shi, and they had some merits under their belt. What achievements did Sima Yan have? Sima Yan had just turned thirty when he came to power. Although he was not young, all the senior officials in the court were his elders. This situation did not change until Sima Yan died. Among the people Sima Yan left to assist his son Sima Zhong, Sima Liang was Sima Yan's uncle, Yang Jun was Sima Yan's father-in-law, and they were both Sima Zhong's grandfathers generation. Wei Guan was the same generation (in-law) as Sima Yan, but Wei Guan is 16 years older than Sima Yan and 39 years older than Sima Zhong. After all, he and Sima Zhong are still two generations apart. This is how Sima Yan has spent his whole life dealing with a group of elders.
Sima Yan had little prestige, and everyone in the court was an uncle here and a senior there, so Sima Yan rewarded ministers much more generously than Sima Zhao. The imperial court has always respected the Three Excellencies, but during the Sima Yan period, the Three Excellencies were no longer enough and became the Eighth Excellencies. In addition, Sima Yan also created a number of dukes and mass-produced a batch of ten thousand households nobles. But can the speed of productivity recovery keep up with the speed of Sima Zhao and his son's reward?
If it had not taken three generations of hard work but only one or two generations to complete the transformation from a powerful regent to an emperor, the problems left by the Western Jin Dynasty would not be so serious.
Second, Sima Yi’s role as an example was too bad.
If the behavior of Cao Cao and his son proves that after the world is in chaos, people with big fists can use the emperor's name to cause trouble and then kick the emperor away, then the behavior of the Sima family proves that this can be done even if the world is not in chaos.
Take the famous Zhong Hui Rebellion as an example. Zhong Hui was born in the Zhong family of Yingchuan. He was 40 years old and was an official Situ. Comparing it with Sima and his son, in terms of background, the Sima clan of Henei is a few blocks behind the Zhong family of Yingchuan. In terms of qualifications, Sima Yi was working as a secretary for Cao Cao when he was 40 years old, Sima Shi was Central-protector of the army when he was 40 years old, and Sima Zhao became the General to conquer the West at the age of 40 thanks to his father and brother's relationship, but they were all inferior to Zhong Hui, who was ranked third in terms of merit behind Sima Zhao and Deng Ai.
Let alone anything else, just the achievment of leading the main force into Hanzhong in the battle to destroy Shu can only be compared to Sima Yi. Zhong Hui's only disadvantage is that he is not as capable as the Sima family. In this case, you Simas and your descendants can usurp the throne , why can't I, Zhong Hui, have any ideas?
However, the consequences of this behavior are quite serious. Although there are a lot of records in history books that someone predicted that Zhong Hui would rebel as an afterthought, when Zhong Hui actually rebelled, Sima Zhao was completely confused at first - I am so good to Zhong Hui, how could he rebel?
When Zhong Hui rebelled, the interrogators had yet to arrive but others were the first to inform on him. The Emperor(Sima Zhao) treated him(Zhong Hui) deeply and did not believe the [rumours].
This reaction is actually very funny. Could it be that the Cao clan treats your Sima family that badly?
Previously, Sima Zhao wanted to increase the number of households for Zhong Hui, but Zhong Hui refused. He also offered Zhong Hui the title of 9 Ministers, but Zhong Hui refused. People at that time called Zhong Hui the reincarnation of Zhang Liang. They not only said that he had many plans, but also praised him for being indifferent to fame and fortune. However, these can all be pretended. When the Cao family awarded rewards to Sima Yi and his sons, didn't Sima Yi and his sons give in and refuse?
But who would Sima Zhao and his son dare to believe in this case? Later, someone falsely accused Shi Bao, who was guarding Huainan of treason. Shi Bao was a confidant general promoted by Sima Shi. He was of low origin and not popular. He was almost an identical match with Deng Ai. Sima Yan was frightened when he heard the news and immediately he was summoned to the court, followed by his dismissal.
Even Jia Chong, the die-hard loser, is still not impossible to rebel. Regarding his opposition to the destruction of Wu, the historian Wang Fuzhi commented that Jia Chong wanted to take the credit of destroying Wu for himself so that he could be the second Sima Yi.
Chong knew that Wu was bound to perish, but he wanted to keep Wu as his own merit. He had been harboring evil ambitions for a long time, and was so afraid of difficulties that he did not dare to raise his voice. The plan to pacify Wu started with Yang Hu, who died, and he promoted Du Yu to finish his work. Since Jia Chong was unable to do it first, and he would not succeed by sharing the merits with others, so he could only block this move until Emperor Wu (Sima Yan) was gone, and then he would serve the country (as regent) and then said that conquest of Wu can succeed today, then all the merits of the generals will be attributed to himself, and there will be no difficulty for him to act as Cao Cao and Sima Yi. Jia Chong even dared to killed the monarch to save himself, but the Simas did not take precautions early, and it would be difficult to avoid chaos. Fortunately, Emperor Wu (Sima Yan) died only seven years after Chong's death. Jia Mi was mediocre and not a blood-stained person, so he was not qualified to serve as Sima Zhao. Otherwise, would the blade that killed Cao Mao not be pointed at the Sima clan? Women and boys are still sufficient enough to destroy the Jin Dynasty, but if Jia Chong was still alive, what the consequences be?
However, it is said that when Sima Yan launched the extermination of Wu, Jia Chong died three years later and he was already half buried, so how could he have such great ambitions. However, wasn't it true that Sima Yi launched the Gaoping Coup only two years before his death? A vivid textbook was placed in front of him. Even if Jia Chong may not think so, would Sima Yan not suspect that Jia Chong thinks so? Therefore, the curse left by Sima Yi has been plaguing his descendants, and the monarchs and ministers are also mutually constantly consumed by suspicion.
Third, the advantage of high fertility has turned into a disadvantage. During the Wei and Jin Dynasties, large gentry clan relied on marriage to consolidate relationships, and some rising stars were able to quickly rise to join the gentry circle and this method was also used. The Sima family has a great advantage in this regard. Sima Yi has 7 brothers, and he himself has 9 sons and 2 daughters. Although Sima Shi has no sons, he has 5 daughters. The most exaggerated one is Sima Yan, who has a total of 26 sons and 13 daughters.
Relying on the strong ability to spread branches and leaves, the Sima family almost formed a marriage relationship with all the high-ranking families at that time. At the same time, the men in the imperial family became good helpers for three generations of Sima's ancestors and grandchildren to seize power. Sima Wang, Sima Liang, Sima Jun and others have all gone out to the border and made contributions.
However, the influence of these royal members are also growing. For example, Sima Fu, who cried out that he was a loyal minister of the Wei Dynasty, was the grandfather generation of Sima Yan. He had made great contributions in destroying Cao Shuang, defeating Zhuge Ke, and deposing Cao Fang, not to mention Sima Fu, even Sima Fu's son Sima Wang is not someone Sima Yan dares to provoke casually. In the end, Sima Fu was named King Anping with 40,000 households, and the ancient post of Dazai was specially set up for him.
In addition, because Sima Shi's younger brother Sima Zhao succeeded him when Sima Shi died, Sima Yan's younger brother Sima You also wanted to take over his brother's duties all day long. People often accuse Sima You of dying because of Sima Yan. In fact, Sima Yan was very kind to Sima You. Sima You's treatment before his death was as follows: King of Qi, Grand Marshal, and military commanders of Qingzhou, plus Palace Attendant, and Jiajie, private camp has a thousand people, and the marshal chariot under the personal riding tent remains as before, and an additional unit is added to promote the army. There are twenty official cavalrymen, and five Sima cavalrymen are installed.
It can be said that he wants power, status, and face, and he had power, status and face. But the only thing missing is refusing to make him his heir. Under this situation, Sima Yan had to give real power to the kings. On the one hand, he learned from the negative example of Cao Wei, and on the other hand, Sima Yan did not have the strength to attack his own huge clan. If Sima Yan wants to launch an attack against the clan, in terms of prestige and legal principles, there is no shortage of princes who can stand up and take the lead in resistance, and there is no shortage of princes who are qualified to inherit Sima Yan's position. If Sima Yan could barely control the situation, what about Sima Zhong who succeeded Sima Yan after his death?
Fourth, the backlash caused by excessive governance.
As mentioned earlier, several generations of the Sima family have given generous rewards in order to tie the interests of the wealthy gentry clans to themselves. Since the Sima Yi period, he has been deliberately trying to win people's hearts, such as setting up the post of Dazhongzheng and giving privileges to wealthy gentries to gain support.
Later, when enemies of Sima Yi was implicated because of rebellion, he often used lenient means to win over people's hearts. For example, during the Gaoping coup, all Cao Shuang's fellow officials were pardoned, no one in Xiahou Ba's family was killed after he surrendered to the enemy, and Cao Wenshu and Guo Huai 's wives were not held accountable, etc. These methods were later learned by Sima Yi's descendants, and the result was that the punishment of high-ranking people became more and more lenient. In the end, this model almost became a common system, and even Zhong Hui could rebel without implicating his entire family (Deng Ai, who was born in a poor family can't).
When it came to Sima Yan, the tolerance of governance reached its peak. As mentioned before, Sima Yan dealt with a bunch of powerful people who were at least ten years older than him and had made great achievements, and behind them were a bunch of prestigious and senior relatives. Under this situation, Sima Yan had no choice but to indulge the ministers. This is an extreme way to gain support. We often see records in history books saying that Sima Yan knew something was wrong with a certain official or that there was a problem with a certain system, but he did not deal with it. For example, Jia Chong has always been firmly opposed to the conquest of Wu, but in order to complete the conquest of Wu without offending Jia Chong, Sima Yan deliberately gave him the title of Commander in Chief for destroying Wu. After Jia Chong became the CIC, he continued to obstruct the action. As a result, by the time after the annihilation of Wu was completed, Sima Yan also gave him 8,000 pieces of silk, increased the number of 8,000 households in his fiefdom, and also rewarded a bunch of his relatives.
The most famous story is that Sima Yan let his senior officials fight for wealth, which immersed the entire Western Jin Dynasty officialdom in an atmosphere of drunkenness and extravegance. However, for Sima Yan, a hundred high-ranking officials fighting for wealth are not as scary as one Zhong Hui who is indifferent to fame and wealth. Shi Chong, a well-known expert in fighting with riches, is the son of the just-mentioned Shi Bao. When Shi Bao was falsely accused of treason, Sima Yan was so frightened that he dismissed his official position. However, when Shi Chong was embezzling and fighting with money and riches, Sima Yan did not accuse him. But as a result, the entire Western Jin officialdom from the central to the local levels was extremely corrupt. The powerful class of the gentry had all kinds of privileges, and the social atmosphere was corrupted. It is obvious that the people at the bottom were squeezed. Therefore, since the Western Jin Dynasty unified China, this regime has no trace of the vitality of a new dynasty.
Liu Bei did nothing wrong.