In 808 AD, in order to select talented individuals of upright character who would dare to speak frankly and remonstrate, Emperor Xianzong of the Tang Dynasty ordered a civil service examination to be held. Li Zongmin and Niu Sengru, officials from humble backgrounds, took this exam and passed with honors. At the time, the Prime Minister was Li Jifu, who came from an aristocratic family. After reading the essays of Li Zongmin and Niu Sengru, he discovered that they had criticized the government and satirized him in their exam papers. Li Jifu, who already looked down on officials of lowly origins, deeply despised Li Zongmin and Niu Sengru and sought to frame them. He told Emperor Xianzong that the reason Li Zongmin and Niu Sengru had passed was due to their personal connections with the examiners. Emperor Xianzong, fully trusting Li Jifu's words, demoted several examiners and did not promote Li Zongmin or Niu Sengru.
This seemingly trivial incident actually marked the beginning of the forty-year-long Niu-Li factional strife.
In 814 AD, Prime Minister Li Jifu passed away. In 820 AD, Emperor Xianzong of Tang was killed by eunuchs, and Emperor Muzong of Tang succeeded the throne. Emperor Muzong promoted Li Jifu's son, Li Deyu, to the position of Hanlin Academician, making him the leader of the aristocratic bureaucrats.
In 821 AD, the imperial court organized a jinshi examination, presided over by Qian Hui, Vice Minister of Rites, with Yang Rushi, a Right Rectifier, serving as the chief examiner. The results showed that Pei Zhuan, son of Prime Minister Pei Du, Su Chao, son-in-law of Li Zongmin, a Secretariat Drafter, and Yang Yinshi, brother of examiner Yang Rushi, all passed. Former Prime Minister Duan Wenchang reported to Emperor Muzong of Tang that the examination was utterly unfair, accusing chief examiner Qian Hui of favoritism to allow Pei Zhuan and others to pass. Unsure of the truth, Emperor Muzong consulted Li Deyu and Li Shen, who confirmed Duan Wenchang's allegations. Angered, the emperor ordered a re-examination, and of the original fourteen successful candidates, only three passed again. Concluding Qian Hui had engaged in malpractice, the emperor demoted him to Prefect of Jiangzhou. Li Zongmin and Yang Rushi were also implicated and exiled to remote posts.
Li Zongmin and Yang Rushi believed that Li Deyu had deliberately framed them, and thus harbored a deep hatred for him. Niu Sengru, who also came from a commoner background, deeply sympathized with Li Zongmin and Yang Rushi's misfortune. As a result, officials from aristocratic families, led by Li Deyu, and officials from commoner families, led by Li Zongmin and Niu Sengru, engaged in fierce conflict.
During the Taihe era of Emperor Wenzong of the Tang Dynasty, Pei Du, a senior minister who had served four emperors, recommended Li Deyu to the emperor, praising his exceptional abilities and claiming he was fully qualified to serve as chancellor. At that time, eunuchs wielded immense power, even influencing major events like the emperor's ascension to the throne. Recognizing this, Li Zongmin secured the position of chancellor by bribing the eunuchs. Li Zongmin frequently praised Niu Sengru in front of Emperor Wenzong and suggested that the emperor transfer Niu Sengru to the capital for an official post. Consequently, Niu Sengru was quickly summoned to the capital, where he served as Minister of War and Co-Chancellor. Thereafter, Li Zongmin and Niu Sengru began appointing their own allies to key positions, pushing members of Li Deyu's faction out to local posts. Li Deyu himself was sent to Xichuan (present-day Chengdu, Sichuan) to serve as military governor.
Xichuan bordered Nanzhao and Tubo, making it a strategically vital region. Before Li Deyu arrived, Xichuan had been invaded by Nanzhao, and its military governor, Du Yuanying, was defeated. The court sent Guo Zhao to replace him, but Guo fell ill upon arrival, leaving Xichuan ungoverned and its people in dire hardship. When Li Deyu took charge, he immediately led his troops to build fortifications to repel Nanzhao's attacks, while also recruiting local civilians into the army, strengthening the Tang forces so much that Tubo and Nanzhao dared not raid again.
In 831 AD, the Tibetan general in charge of Weizhou (located in present-day Li County, Sichuan) surrendered to the Tang army. Li Deyu sent troops to occupy Weizhou and reported this good news to the imperial court. Weizhou, originally Tang territory that had been under Tibetan control for forty years, was now returned to the Tang dynasty, and Emperor Wenzong was overjoyed upon hearing this. At that time, a Tibetan envoy seeking peace was in Chang'an, and most court officials believed Li Deyu had acted correctly, advising Emperor Wenzong to maintain friendly relations with Tibet.
However, Niu Sengru, jealous of Li Deyu's great achievement, actually disregarded national interests and requested Emperor Wenzong of Tang to return Weizhou to Tubo (Tibet) and hand over the surrendered Xida and others to Tubo for punishment. As a result, Weizhou, a territory just recovered, was reoccupied by Tubo, and Xida and the others were executed by Tubo.
This incident infuriated Li Deyu, and from then on, the factional struggle between Li Deyu and Niu Sengru intensified. Later, Emperor Wenzong of Tang believed Li Deyu's actions were entirely correct, so he reappointed Li Deyu as Minister of War. Li Zongmin, fearing that Li Deyu might become Prime Minister, spoke many slanders against Li Deyu in front of Emperor Wenzong.
In 840 AD, Emperor Wenzong of the Tang Dynasty passed away, and Emperor Wuzong ascended the throne. Li Deyu finally had his chance to turn things around and was appointed as Prime Minister. After taking power, Li Deyu began to sideline Li Zongmin, Niu Sengru, and others.
Li Deyu was a highly capable individual who enjoyed the deep trust of Emperor Wuzong of the Tang Dynasty. At that time, the Uighurs were defeated by the Kyrgyz and migrated southward. Li Deyu advised Emperor Wuzong to both provide relief to the Uighurs and send people to guard against them strictly. Later, the Uighurs plundered and committed many atrocities north of Yunshuo (present-day northern Shanxi). Li Deyu dispatched Zhang Zhongwu, the military governor of Youzhou (present-day Beijing), and Liu Mian, the military governor of Hedong (present-day southwest of Taiyuan, Shanxi), to jointly attack the Uighurs, achieving a great victory.
After this, Liu Congjian, the military governor of Zelu (present-day Changzhi, Shanxi), passed away, and his nephew Liu Zhen attempted to take over the position. Li Deyu dispatched troops to suppress the rebellion, and after a year of effort, finally pacified Luzhou.
In 846 AD, Emperor Wuzong of the Tang Dynasty died, and Emperor Xuanzong ascended the throne. Emperor Xuanzong strongly disliked Li Deyu's autocratic and monopolistic power, so he demoted Li Deyu to a local official post, and other officials of Li's faction were also demoted. Li Zongmin, Niu Sengru, and others were once again reinstated to important positions.
In 848 AD, Niu Sengru died of illness. In 849 AD, Li Deyu died while serving in Yazhou (administrative seat located southeast of present-day Qiongshan, Hainan). Shortly after, Li Zongmin also died. Thus, the "Factional Strife" that had lasted for forty years came to an end.