In the early years of the Western Jin Dynasty, Emperor Sima Yan of the Jin Dynasty enfeoffed 27 kings with the same surname, taking the county as the country. Since then, the power of kings has been expanding, and they can even select and appoint civil and military officials at home and levy taxes on the state.
The king occupies an important position in the imperial court, with both executive power and a considerable number of troops. These powerful vassals were only the fuse to destroy the unification of the Western Jin Dynasty.
Under the supervision and control of Emperor Wu of the Han Dynasty, the political situation remained stable. In 290 AD, Emperor Wu died and Emperor Jinhui succeeded to the throne. Hui Di is insane and unable to govern the country.
Later, because of the Eight Kings Rebellion between the consorts and the Queen Jia, the Eight Kings attacked each other and destroyed the great country of the Western Jin Dynasty.
Extended data:
The Eight Kings Rebellion, which lasted for 16 years, greatly destroyed social production. During the reign of Taikang, there was a prosperous scene of "the rule of Taikang" in the society, and the rebellion of the Eight Kings made the people suffer from war.
The princes attacked each other and killed each other. Wherever they went, they plundered and destroyed water conservancy projects, displaced people, and suffered 300,000 casualties. This war directly weakened the rule of the Western Jin Dynasty and gradually disintegrated its regime.
In the struggle for power and interests, kings also used minority forces as foreign aid. For example, Sima Ying led the Xiongnu soldiers from Liu Yuan to participate in the melee. Sima's flesh and blood cannibalized each other, and the leaders of ethnic minorities took the opportunity to enter the fortress, and the Yongjia rebellion followed, and the Western Jin Dynasty soon perished.
References:
Baidu Encyclopedia-Western Jin Dynasty