During the Three Kingdoms period, why did Sun Quan marry his sister to Liu Bei, which led to the deterioration of the relationship between the two sides?

Even though Sun Quan married his sister to Liu Bei, the relationship between Sun and Liu eventually broke down. In fact, this is related to the interests of Sun and Liu, from the close relationship between the two countries before to the return of Jingzhou three counties and the marriage of Sun Quan's sister. Finally, it led to a bad relationship.

first of all, from the attitude of Liu Bei, the western Shu? Both Liu Bei and Zhuge Liang just want to destroy Cao Cao, and the Han Dynasty can flourish. Strategically, it did not involve how to deal with Sun Quan. It can be seen that his attitude actually has a certain degree of contempt for Sun Quan. Later, they put forward the strategy of United Sun and refusing Cao Cao because they had to do it themselves.

After Shu and Wu passed through Battle of Red Cliffs, and Wu Lu Su and others also advocated maintaining good relations with Liu Bei, the relationship between the two families became closer. Later, Shu borrowed Lingling, Guiyang and Changsha counties of Wu State. However, Shu attached great importance to these three counties and refused to return them. Later, in order to maintain the friendly relations between the two countries, Sun Quan betrothed his sister to Liu Bei.

But Sun Quan's arrogant sister didn't feel her responsibility. Liu Bei didn't feel tender either, so he had doubts about Sun Quan's move. And take precautions. Sun Quan's sister felt bored and returned to the State of Wu. And Liu Bei and Zhuge Liang made Zhao Yun recapture Sun Quan's sister, which made Sun Quan lose his dignity and laid a hidden danger for the later breakdown of relations.

Later, the relationship between the two sides broke down because of the return of Jingzhou, and Guan Yu was rude to the envoys sent by the State of Wu. After that, the strategy of uniting Wu against Cao basically ceased to exist. Shu made many mistakes in tactics. In the end, Guan Yu was defeated by Lu Xun, a famous Soochow player. The relationship between the two sides has completely broken down.