thumb|Inscription by Yu Di and his escorts, Tenth year of Zhenyuan, 794 AD. Located in Xialongtan, Xiangying Mountain, [[Deqing County, Zhejiang|Deqing County. It is now the county's cultural relics protection unit.]]
Yu Di (; died 818), courtesy name Yunyuan (允元), formally initially Duke Li of Yan () and later Duke Si of Yan (), was a Chinese diplomat, military general, politician, and warlord during the Tang dynasty. He was a powerful warlord at the end of the reign of Emperor Dezong but submitted to imperial authority during the reign of Emperor Dezong's grandson Emperor Xianzong.
Background and early career
It is not known when Yu Di was born, but it is known that his family was from Henan Municipality (, i.e., the region of the Tang dynasty eastern capital Luoyang). His family traced its ancestry to the prominent Xianbei clan Moniuyu () of Northern Wei, which changed its name to Yu when Emperor Xiaowen of Northern Wei had the Xianbei clans' names changed to Han names. Among Yu Di's ancestors were generals and officials of Northern Wei, Western Wei, Northern Zhou, Sui dynasty, and Tang, including the prominent Northern Zhou general Yu Jin. Yu Di's grandfather Yu Wang (于汪) served as the director of the archival bureau, while his father Yu Xiong () served as a military advisor to a prefectural prefect.
It was said that because of his heritage, Yu Di was made an imperial guard officer at the start of his official career, and then as the sheriff of Huayin County (, in modern Weinan, Shaanxi). The surveyor of the circuit, Liu Wan (), later invited him to serve as assistant; yet later, while he was serving as the secretary general of Yueyang<!--cf. Zdic--> County (, in modern Xi'an, Shaanxi), he served under the senior official Zhang Yi at a treaty signing with the Tibetan Empire. Thereafter, he served as Simen Yuanwailang (), a low-level official at the ministry of justice (, Xingbu), as well as Shiyushi (), a low-level imperial censor. He later again served on a diplomatic mission with the Tibetan Empire, and was said to have served well and been highly regarded as a result. He later served as the magistrate of Chang'an County (one of the two counties making up the Tang capital Chang'an) and then as Jiabu Langzhong (), a supervisorial official at the ministry of rites (, Libu).
Thereafter, however, Yu grew arrogant, and it was said that he recruited more soldiers and acted as if the land south of the Han River was all his domain. It was also said that he killed people at will. As Emperor Dezong was more interested in appeasing the regional governors at the time, he acted as Yu requested, including promoting Xiang Prefecture's status into one where a commandant would be stationed, just as the capital of two other circuits ruled by warlords ruling their realms independently from the imperial government and Wei Prefecture (, in modern Handan, Hebei), the capital of Weibo Circuit (), then ruled by Tian Xu. On one occasion, he falsely accused his subordinate Yuan Hong () the prefect of Deng Prefecture (, in modern Nanyang, Henan) of corruption. After Emperor Dezong ordered that Yuan be exiled to Duan Prefecture (, in modern Zhaoqing, Guangdong), Yu seized Yuan from the eunuch sent to escort Yuan to exile, forcing the eunuch to flee back to Chang'an, and then submitted a petition complaining that Yuan was punished too harshly, and only after Emperor Dezong changed the punishment to demotion to be the secretary general of Ji Prefecture (, in modern Ji'an, Jiangxi) did Yu allow Yuan to leave. On another occasion, when Yu became angry with his assistant Xue Zhenglun (), he requested Emperor Dezong to demote Xue to be the secretary general of Xia Prefecture (, in modern Yichang, Hubei), but after Emperor Dezong issued the edict, Yu was no longer angry with Xue by that point, and submitted another petition to keep Xue as assistant. Emperor Dezong agreed with that request as well. It was also said that he, without imperial permission, stationed troops at the important city of Nanyang, causing the imperial government such distress. Shortly after, Emperor Xianzong had other officials hint to Yu that he should personally pay homage to the emperor to thank him, a suggestion echoed by Yu Di's son Yu Fang (). Further, when he suggested to Emperor Xianzong that harsher punishments be instituted, Emperor Xianzong did not accept the proposal and further, in discussion with other chancellors, referred to Yu as being evil in submitting the proposal and wanting him to lose the hearts of the people.
Yu Di was not happy serving in this manner. In or shortly before 813, there was a man named Liang Zhengyan () who claimed to be a close relative of the powerful eunuch Liang Shouqian (), and Yu had his son Yu Min () bribe Liang Zhengyan, seeking to be made a military governor again. However, at a later point, Liang Zhengyan was revealed to be a fraud, and Yu Min demanded the return of the bribe, which Liang Zhengyan refused. In anger, Yu Min trapped Liang Zhengyan's servant by trick, and killed and dismembered the servant. When this was discovered, Yu Di and Yu Jiyou went to the palace gate in mourning clothes to seek mercy, but the palace guards refused to let him meet the emperor. He was subsequently demoted to be the assistant to Emperor Xianzong's granduncle Li Lian () the Prince of En, and he was denied further access to the emperor's presence. Yu Min was exiled to Lei Prefecture (, in modern Zhanjiang, Guangdong), while Yu Di's other sons were all demoted in rank. On the way to exile, Yu Min was ordered to commit suicide, while Liang Zhengyan and his associate, the Buddhist monk Jianxu () were executed. Later that year, Yu Di was elevated to the higher position of Taizi Binke (), an advisor to Emperor Xianzong's crown prince Li Heng. many generals submitted their wealth to support the campaign. When Yu tried to submit a large amount of silver and gold, however, Emperor Xianzong refused to accept his submission. In 818, Yu requested retirement, and traditionally, when an official sought retirement, he would be allowed to retire at a greater position than he actually held. The chancellors thus proposed giving him the greater position of Taizi Shaobao ()—also advisor to the crown prince, but at higher position. Emperor Xianzong disapproved and had him retire as Taizi Binke. He died later that year and was given posthumous honors but the posthumous name Li (, "cruel"). After Emperor Xianzong's death and succession by Li Heng (as Emperor Muzong) in 820, Yu Jiyou, on an occasion when he attended a hunt with Emperor Muzong, pleaded with Emperor Muzong to give his father a more honorable posthumous name. Many officials opposed such a request, but Emperor Muzong eventually gave Yu Di the posthumous name of Si (, "repentant").