In 762 AD, Emperor Xuanzong of Tang died of illness, Emperor Suzong died of shock during a palace coup, and the throne passed to his son Li Yu, who became Emperor Daizong of Tang.
In 765 AD, Pugu Huai'en, the Deputy Commander-in-Chief of Hebei who had distinguished himself in battle during the An Lushan Rebellion, grew dissatisfied with the imperial court's rewards, believing his military achievements far exceeded what he had received, and thus conceived the idea of rebelling against the Tang Dynasty. He contacted the Tubo and Uighur tribes, deceiving them by claiming that Guo Ziyi had been killed, and urged them to send troops to attack the Tang. Guo Ziyi was a renowned Tang general who had repeatedly led forces against the Tubo and Uighur in battle, and these tribes had often planned to rise against the Tang but always abandoned the idea out of fear of Guo Ziyi. Hearing that Guo Ziyi was dead, they were overjoyed and immediately dispatched their armies to march on the Tang Empire.
In early October, the troops of Tubo and Huihu had already advanced to Jingyang (present-day Jingyang, Shaanxi) north of Chang'an, placing the capital in grave danger. The eunuch Yu Chao'en, who two years earlier had urged Emperor Daizong of Tang to flee when Chang'an was attacked by Dangxiang and Tubo forces, once again advised the emperor to abandon the city. Facing strong opposition from court ministers, Emperor Daizong decided to dispatch Guo Ziyi to lead the army in resisting the Tubo and Huihu offensive.
After receiving the order, Guo Ziyi immediately rushed to Jingyang. Due to insufficient troops in Jingyang, he ordered his soldiers to build defensive fortifications to guard against enemy attacks while also sending scouts to gather intelligence on the enemy forces.
Pugu Huai'en unexpectedly fell ill and died on the march, and although the Tibetans and Uighurs were allied forces, they only cared about their own interests; originally, with Pugu Huai'en mediating between them, they could get along harmoniously, but once he died, they each fought for themselves, with neither side willing to follow the other's command.
This important intelligence greatly excited Guo Ziyi, as if he could already see the dawn of victory. He believed that the Tibetans and Uighurs were strong and formidable, and if he simply held the city and defended it, it would be very difficult to hold out; but if he could divide the Tibetans and Uighurs, their strength would be greatly diminished, and perhaps he could make them withdraw without using a single soldier. He recalled that during the An Lushan Rebellion, the Uighur general Yaogelo had led his troops to fight alongside the Tang army under his command against the rebels, and they had some rapport, so he decided to win Yaogelo over to his side. Thus, he sent his subordinate Li Guangzan and others to the Uighur camp to meet with Yaogelo.
Li Guangzan said to Yaogeluo: "Lord Guo (a respectful title for Guo Ziyi) sent me here to ask you this: the relationship between the Huihu and the Tang Dynasty has always been excellent, so why have you heeded the words of petty individuals and launched an attack on our great Tang?"
After hearing Guo Ziyi's name, Yaogeluo was very surprised and asked, "Is Lord Guo still alive? Where is he?"
When Li Guangzuan told Yaogelo that Guo Ziyi was in Jingyang, Yaogelo simply did not believe it, because Pugu Huaien had told him that Guo Ziyi was already dead. To prove that Guo Ziyi was indeed alive, he demanded to see him.
After Li Guangzan returned to the military camp, he reported this matter to Guo Ziyi. Guo Ziyi said, "Given the current situation, our forces are insufficient while theirs are strong and well-equipped. If we try to win through force, it will be extremely difficult. Our Tang Dynasty has always had good relations with the Huihu. If I go to see Yaogeluo in person, there's a chance I can persuade him to withdraw his troops."
After hearing this, the generals all agreed it was an excellent plan, but they worried for Guo Ziyi, as going to the enemy's camp was extremely dangerous. Some suggested he bring five hundred elite cavalry with him, but Guo Ziyi refused, saying that just a few men would be enough.
Just as Guo Ziyi was about to mount his horse and set off, his son Guo Xi blocked his path and said, "You are now the Grand Marshal of the Great Tang, bearing the nation's safety on your shoulders, while those men are wolves and tigers. How can you disregard your own safety and venture into a den of wolves and tigers?"
Guo Ziyi replied: "Looking at the military strength of both sides, we are at a clear disadvantage. If we engage in battle, we will surely be defeated. At that point, not only would our lives as father and son be in danger, but the entire nation would also suffer. This time, when I meet with the Uighur general, I will speak with them openly and honestly. If they agree to withdraw their troops, the country can avoid war, and perhaps the Uighurs will even join us in dealing with the Tibetans—that would be a blessing for the nation. For the sake of the country, I believe this is well worth doing. Moreover, even if I meet with misfortune, aren't you all here holding the line?"
After saying this, Guo Ziyi led a few cavalrymen and galloped toward the Uighur military camp. As they neared the camp, the cavalrymen, following Guo Ziyi's instructions, shouted in unison, "Lord Guo has arrived! Lord Guo has arrived!"
Upon hearing the shouts, the Huige soldiers immediately reported to Yaogelo. Greatly startled, Yaogelo ordered his soldiers to draw their bows and prepare for battle. Not far from the Huige military camp, Guo Ziyi threw his spear to the ground, removed his armor, and led his horse toward the Huige camp. After Yaogelo and the Huige generals clearly saw Guo Ziyi's face, they all exclaimed, "Lord Guo, that man is truly Lord Guo!" Then they stepped forward one after another to welcome Guo Ziyi.
Guo Ziyi said to Yaoluo Ge: "During the campaign to suppress the An-Shi Rebellion, your Uyghur people rendered great service to the Tang Dynasty, and the Tang Dynasty rewarded you generously in return. Why then do you violate the treaty between Tang and the Uyghurs by helping Pugu Huai'en rebel against the Tang? I have come here today in the hope that you will stop making such foolish mistakes. I came alone, having already set aside all thoughts of life and death. If you wish to kill me, then do so without delay. Even if I die, my troops will never allow you to advance even a single step!"
Yao Geluo said with great shame, "Lord, when you speak like this, you are practically mocking us. Even if we had the audacity of heaven, we would not dare to harm a single hair on your head! If Pugu Huai'en had not told us that the Tang Emperor and Lord Guo had both died, how would we have dared to come here? Now that we know the Tang Emperor is still in Chang'an and Lord Guo is stationed here, how could we possibly fight against you?"
Guo Ziyi was quite satisfied with Yaoge Luo's response, and hoping Yaoge Luo would help the Tang army fight against the Tibetan forces, he said: "The Tibetans, disregarding their kinship ties with the Tang Dynasty, have been burning, killing, and looting along the Tang border, seizing vast amounts of wealth. As a Tang general, I simply cannot tolerate the Tibetans' actions, and I am determined to drive them back. If you can help us repel the Tibetans, then the spoils they have plundered will be yours."
Yao Geluo said, "We were deceived by Pugu Huaien and did things that wronged you, Lord. Now we are willing to atone for our mistakes by joining you in fighting against the Tibetans."
When the Huige soldiers heard the conversation between Guo Ziyi and Yaogeluo, they unconsciously gathered around them. Guo Ziyi's attendants, worried that they might harm him, slowly moved closer to his side. Guo Ziyi waved his hand, signaling them to step back, and then asked Yaogeluo to bring out the wine. Yaogeluo's servant poured a cup of wine for both Guo Ziyi and Yaogeluo. Guo Ziyi raised his cup, poured the wine onto the ground, and swore an oath: "Long live the Tang Emperor! Long live the Huige Khan! From now on, if anyone violates this covenant, may they die without a place to be buried." Then, Yaogeluo also sprinkled wine and swore an oath, and the two sides signed a peace treaty.
The Tibetan generals, hearing that Guo Ziyi had signed a peace treaty with the Uighurs, feared a joint Tang-Uighur attack and fled that night.