QUOTE
In 756, the An Lushan Rebellion had broken out and the Tang court desperately needed to get the Uyghurs on their side to fight An Lushan. So when they sent the Prince of Dunhuang, Li Chengcai to get Uyghur support, and the Uyghur kaghan married his daughter to Li Chengcai, not only did Tang Xuanzong accept this but he also gave the Uyghur girl the title of Princess of Pijia.
As for real Chinese princesses being sent to the Uyghurs, I have not heard much of this. Could you give some details?
OK, I went to check this up in my reference book. When the Uyghur kaghan married his 'daughter' to Li Chengcai in 756, this 'daughter' was actually a sister of his queen whom he had taken as an adopted daughter for the purpose of the marriage alliance with the Tang. The Pijia (Bilge) in the title given by the Tang to this 'princess' was a traditional title among the Uyghur.
In 758, after the Uyghur had helped the Tang to retake Luoyang and Chang'an, Emperor Suzong (Li Heng) married his youngest daughter, Princess Ningguo, to the Uyghur kaghan (Yingwu Kaghan). The princess was given the title of kedun (Queen) by the kaghan. This was the first time in history that a Chinese emperor gave his own daughter as a bride to a ruler of a foreign ethnicity. In fact, the daughter of Prince Rong (a member of the imperial house) also went along as a concubine for the Yingwu Kaghan.
In 759, the Yingwu Kaghan passed away. Princess Ningguo, widowed after just a year of marriage, chose to return to the Tang court. But the daughter of Prince Rong stayed on and married the next kaghan (Yingyi Kaghan, son of Yingwu Kaghan), being raised to the rank of kedun and addressed as the Lesser Princess Ningguo. After the Yingyi Kaghan was killed by his general Tun Bagha Tarkhan in 779, the Lesser Princess lived alone as a widow and died in 791.
The Yingyi Kaghan also married two daughters of the great Tang general Pugu Huai'en (of Tie'le ethnicity): one in 758 when he was still heir to the kaghanate (she was given the kedun title along with the Lesser Princess Ningguo when he became kaghan in 759), and one in 768 after the first daughter passed away. It's interesting that by 768, Pugu Huai'en had died and was considered a rebel. He had been accused of conspiring with the Uyghurs in 763, and in his anger rose in rebellion with the Uyghurs, Tibetans, Tuyuhun and Tanguts but suddenly died of illness in 765. Guo Ziyi then won the Uyghurs back to the side of the Tang. But Pugu Huai'en's youngest daughter was still given the title of Princess Chonghui by the Tang court and married to the Uyghur kaghan in 768 - probably because Huai'en's prestige among the Uyghurs was so high.
Tun Bagha Tarkhan took over as the Uyghur ruler, but did not have official recognition from the Tang court. Hence in 787, he sent an envoy to the Tang court to request a Tang princess in marriage. Emperor Dezong was persuaded by his prime minister Li Mi to agree to this, and sent his eighth daughter Princess Xian'an to be Tun Bargha's kedun in 788, on five conditions including that Tun Bargha declared himself the 'son' and subject of the Tang. Tun Bargha was also given the title of Tianqin Kaghan.
The Tianqin Kaghan died in 790, and Princess Xian'an had to marry three more kaghans in succession: the Zhongzhen Kaghan (790-795), the Fengcheng Kaghan (795-805), and the Huaixin Kaghan (805-808). The Zhongzhen Kaghan was killed and overthrown in 795, so the Princess married a general of the Ediz tribe who became the Fengcheng Kaghan. She lived among the Uyghur 21 years, dying in the same year as the Huaixin Kaghan. It is often forgotten in Chinese histories that princesses like her who were married off to Central Asian rulers often had to observe the custom of the land and marry the son or the successor upon the death of their husband. This happened to Liu Jieyou, the Wusun Princess during the time of Han Wudi, and also the famous Wang Zhaojun.
After Princess Xian'an's death, the Uyghurs requested another princess from the Tang. This was turned down until Emperor Xianzong finally agreed in 820 because the Tibetans were a constant threat and the Tang needed to keep the Uyghurs from allying with the Tibetans. However, Xianzong died before this could take place. His son who succeeded him (Emperor Muzong) then had his tenth sister given the title of Princess Taihe in 822 and sent to be the kedun of the Chongde Kaghan. This was the last Tang-Uyghur royal marriage before the destruction of the Uyghur kaghante by the Khirghiz in 840.
In 835, Princess Taihe actually sent the Tang court a gift of seven female Uyghur horse-archers, and two Shato Turk slave boys! In 841, after the Uyghur Baimei Kaghan was killed by the Khirghiz, the last Uyghur kaghan Wujie Kaghan (who had been nominated to the position by the various chiefs) fled to the Tang, bringing Princess Taihe with him.