Since then, the Liao State has become a world empire across East Asia and the hinterland of Central Asia, and is known for its strength and stability, and the Song Dynasty stands side by side, because the strength of the two countries is almost equal, it is since the Jiaxing period, the two countries have no boundaries, and the arrival of peace has made the border trade between the two countries extremely prosperous, far beyond the previous generation, and led to the rapid development of the economy of the two countries.

And it was the excessive economic prosperity that caused the Song and Liao to quickly corrupt the two countries, and as a result, gave the rising Mongols an opportunity.

Under the continuous influence of the Song Dynasty, the rapid sinicization of the Liao State was more thorough than before, like the Liao Xiangzong Yelu Zhilugu admired the thin gold calligraphy of the "Artist Emperor" Song Huizong Zhao Ji, and repeatedly asked the Song Dynasty for the true handwriting of Song Huizong, and made great efforts to imitate it, so that it reached the point of perfection. Although he did not boast of being a literati emperor, he built a large amount of land in the case of sufficient financial resources of the Liao State, and built a garden resort called "Xiyue" in the Datong Mansion of Xijing, which was called "Hanging Garden" and "a beautiful beauty only in heaven" by Western merchants and envoys who came to Xijing.

The so-called upward and downward effect, in the later period, the ministers of the Liao State completely lost the fierce and rough atmosphere of Yelu Abaoji, Xiao Yanyan, Yelu Dashi and others who created the hegemony of the Great Liao Empire, and became polite and polite. When the cabinet meeting was convened, a group of Khitan aristocratic ministers spoke in a low voice, with elegant words, and were humble to each other without any disputes, which was called "maintaining the demeanor of the prime minister" (Yangxiangti) at that time. When it was necessary to report to the emperor a major event such as a flood, drought, or civil suffering, he said, "I am afraid that the Holy Lord will be in trouble, so we should not report it to him rashly," and when a decision had to be made, he said, "We will discuss it at the next meeting." If there is a young man who does not know the depth and wants to carry out reforms, they will send him to do one or two extremely difficult and unfamiliar things, and then take the opportunity to remove him and then select someone who is soft and familiar and has no edge and is easy to control to become an official. In the last years of the Liao Kingdom, every time the northern Mongol troops pressed the border, the monarchs and ministers sat in the court, relatively crying. When the Mongols had finished plundering and retreated to the north, they held a feast, drank in the high pavilion, and happily celebrated and survived a difficulty.

Like the Liao State, the Song Dynasty gradually returned to the state of Huizong Xuanhe in the last years, which was better than that of the Liao State. After the death of Wenzong Zhao Kai, known as "Xiao Yao Shun", the Song Dynasty began to accelerate its decline.

After Zhao Kai's death, it was supposed to be the crown prince Zhao Tai who ascended the throne, but at this time Zhao Tai was young, and the prime minister Zhang Jun said that the Empress Dowager Meng set up Zhao Kai's younger brother Kang Wang Zhao Gou to ascend the throne for Gaozong.

Soon after Gaozong ascended the throne, the Mongolian army attacked Liao, at this time, someone in the court proposed to contact Mongolia to jointly attack Liao, restore the hometown of the Han and Tang Dynasties, Prime Minister Zhang Jun wanted to report the humiliation of the defeat of the year, and insisted on pampering Zhao Gou, Zhao Gou agreed to send troops, and ordered Prime Minister Zhang Jun to be fully responsible for the Liao invasion, so Zhang Jun sent envoys to the north to contact the Mongols, about to attack Liao, divide its land equally, while transferring 600,000 troops, transporting 400,000 stones, ordering Yue Fei, Di Feng, Han Shizhong as generals, and the troops were divided into three northern expeditions.

For Lianmeng's invasion of Liao, Yue Fei, Di Feng, and Han Shizhong clearly expressed their opposition at the beginning, believing that Lianmeng's attack on Liao was like the destruction of Liao by Lianjin in the past, and after the Liao was destroyed, Jin attacked Song, so that Song was destroyed by Jin. After receiving orders from the imperial court, they wrote one after another to express their opposition.

Song Gaozong Zhao Gou and Zhang Jun were very annoyed by the opposition of the three generals, and successively issued edicts and reprimands, ordering them to prepare for the attack on Liao within a time limit. Subsequently, Di Feng and Han Shizhong also submitted their resignation applications to the imperial court.

Seeing that the three generals were so disobedient, Zhao Gou was furious, quickly dismissed the three generals, and exiled Yue Fei to Qiongzhou (Hainan Island) on the charge of "dereliction of duty", Di Feng sent to Qingzhou, where his ancestor Di Qing was demoted, and Han Shizhong was demoted to Zhangzhou.

After the three generals were dismissed and punished, Zhang Jun recommended Zhang Jun, Liu Qi, Wei Sheng, and Li Xianzhong as generals to replace the three generals to lead the troops and horses, of which Zhang Jun was the main commander, and Liu Qi, Wei Sheng, and Li Xianzhong were divided into three armies to attack Liao. Due to the sudden departure of Yue Fei and other three major generals, the soldiers were frightened, and Zhang Jun, who was defeated by the Liao army in the past, became the main general again, and as a result, all the generals were not convinced, and the army's morale fluctuated, and a deep crisis was buried from the beginning.

The Song army went north to attack Liao, at this time the Liao army has been corrupted, the generals of the past years such as Xiao Hulila, Yelu Songshan, Yelu Yanshan and other generals have died, the Song army has won a great victory all the way, only in the attack on Liaoxi Beijing encountered the strong resistance of the Liao army led by the Empress Dowager Yelu Pu Suwan, not only the siege could not go down for more than a month, but suffered heavy casualties.

When the two armies of Song and Liao were exhausted, the Mongol army arrived and launched a fierce attack on Xijing, and after a month of fierce and cruel fighting, Xijing was finally captured, Yelu Pu disappeared in the chaos, Liao Emperor Yelu Zhilugu went to Xiyue Garden to set fire to himself, and the noble ministers went with him to the dead, and the Liao State perished.

After the capture of Xijing, the Song army and the Mongol army attacked various important towns in the Liao State separately, which lasted for a year, except for Central Asia, all parts of the Liao State were pacified, and the Liao State was destroyed.

After the fall of the Liao State, the Song and Mongolia sides divided the Liao land equally according to the agreement, and as a result, for the proposal of the Song Dynasty, Mongolia thought that it was a loss, and the two sides have not been able to negotiate, in this case, the belligerent Zhang Jun simply secretly worked out a plan to attack the Mongolian army's big tent with the generals, and made an appointment to march together, the result was that the Mongolian army found out, secretly set up an ambush, first annihilated Li Xianzhong's troops who came to attack, and then concentrated their forces to attack Wei Sheng's army, after Wei Sheng's army was destroyed, the Mongolian army turned its head to attack Liu Qi's army, Liu Qi got the warning of Wei Sheng's army in advance, and formed a formation to defend himself, block the way of the Mongolian army, and ask Zhang Jun for help, Zhang Jun learned that Li Xianzhong and Wei Sheng were killed by the Mongolian army, and the soul was not possessed after the destruction of the whole army, and led the army to flee south, regardless of Liu Qi's life or death, as a result, Liu Qi's army was broken by the Mongolian army's iron cavalry after holding on for more than 20 days, Liu Qi and most of his subordinates were killed, and only more than 1,000 horsemen escaped.

After easily annihilating the main force of the Song army, the Mongol army marched southward, not only expelling the Song army from the land of Liao in one fell swoop, but also attacking the Song Dynasty itself.

After the news of the defeat came, Song Gaozong Zhao Gou was shocked, he put all the blame on the prime minister Zhang Jun, dismissed Zhang Jun, took Zhu Shengfei as the prime minister, reactivated the three generals Yue Fei, Di Feng, and Han Shizhong, restored their official positions, and wanted to use them to save the situation. After receiving the imperial edict, the three generals all went directly to their posts from the place of exile.