Why did the story of September, the most beautiful fairy tale in China, surrender to attack the Tang Dynasty without fighting?

During this period, Uighurs did not always obey the Tang Dynasty. During the Kaiyuan period, Uighurs gradually flourished. It invaded Liangzhou and was repelled by the famous Guo Zhiyun. Therefore, it is equivalent to being raised and taught by the Tang Dynasty, and there is a sense of awe in the Tang Dynasty from the bottom of my heart. Later, after the Anshi Rebellion, Pu Baohuai rebelled against the Tang Dynasty, colluded with Tubo and Uighur 200,000 troops to invade and occupy Chang 'an. Fortunately, Guo Ziyi was there, scaring Tubo back to Chang 'an, and riding into Uighur camp alone to persuade Uighur. The two sides pursued Tubo, and the Tang cavalry fought side by side with the Uighur cavalry and won a great victory.