Wu Zetian was the only female emperor in Chinese history, and she gave birth to four sons. Li Hong, the eldest son, was proficient in government affairs, but unfortunately contracted tuberculosis and died at the end of his 19-year term as crown prince. There is controversy in history about whether he was poisoned by his mother.

Li Xian, the second son, was made crown prince after Li Hong's death, but he was dissatisfied with his parents' way of governing the country and constantly angered his mother. Later, he was involved in a conspiracy case, was deposed as crown prince, and exiled to Bazhou. He was forced to commit suicide shortly afterwards, at the age of only 29, and was later known as Zhanghuai Crown Prince.

Li Xian and Li Dan were Wu Zetian's third and fourth sons, but both were intellectually and temperamentally mediocre and cowardly. However, they both had a remarkable experience: becoming emperor twice.

In 683 AD, Li Xian became crown prince the day after Li Xian was deposed, and became Tang Zhongzong in history. After being deposed, he wandered in solitude for 14 years with his wife Wei. In 698 AD, Wu Zetian recalled him and made him crown prince again. In 705 AD, when Wu Zetian was seriously ill, the prime minister Zhang Jianzhi and others launched a coup and supported Li Xian to become emperor again. However, in the fifth year, Li Xian was poisoned to death by his wife Wei and daughter Anle Princess in Shenlong Palace.

Subsequently, Wei made Li Xian's fourth son Li Chongmao emperor and took control of the government. However, Li Dan's son Li Longji and Princess Taiping launched a coup, eliminated the Wei faction, deposed Li Chongmao, and supported Li Dan to become emperor again.

Although Li Xian and Li Dan both became emperor twice, in the face of their mother Wu Zetian's strong power, they were just puppets manipulated by others.

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