Emperor Cheng of Jin (; December 321 or January 322 – 26 July 342), personal name Sima Yan (), courtesy name Shigen (), was an emperor of the Chinese Eastern Jin dynasty. He was the eldest son of Emperor Ming and became the crown prince on April 1, 325. During his reign, the administration was largely dominated by a succession of regents—initially his uncle Yu Liang, then Wang Dao, then the joint administration of He Chong and another uncle Yu Bing (). He became emperor at age four, and soon after his accession to the throne, the disastrous rebellion of Su Jun weakened Jin forces for decades.
Family background
Sima Yan was born as the oldest son of Emperor Ming of Jin, who was crown prince at that time, by his wife Crown Princess Yu Wenjun, in December 321 or January 322. After Emperor Ming took the throne in 323 following the death of his father Emperor Yuan, he created Crown Princess Yu empress, but did not immediately create Prince Yan crown prince, until 325.
In fall 325, Emperor Ming grew ill. He entrusted the four-year-old Crown Prince Yan to a group of high-level officials, including Sima Yang () the Prince of Xiyang, Wang Dao, Bian Kun (), Chi Jian, Lu Ye (), Wen Jiao, and Empress Yu's brother Yu Liang, perhaps intending that they lead by group with a balance of power. He died soon thereafter. Crown Prince Yan took the throne as Emperor Cheng.
Reign
Yu Liang's regency
Initially, the officials were in charge together, but as Empress Dowager Yu became regent, Yu Liang became effectively the most powerful official in the administration. He changed from the lenient policies of Wang (who was prime minister during Emperor Ming's reign) to stricter applications of laws and regulations, which offended the officials accustomed to Wang's lenience. Further, he became apprehensive of the generals Tao Kan and Zu Yue—neither of whom was mentioned in the list of honors and promotions announced by Emperor Ming's will and believed that Yu had erased their names from the will—and Su Jun, who had allowed many criminals to join his army. In 326, he alienated public opinion by falsely accusing Sima Yang's brother Sima Zong () the Prince of Nandun of treason and killing him and deposing Sima Yang.
The Su Jun Disturbance
In 327, apprehensive of Su, Yu decided to try to strip his military command by promoting him to the minister of agriculture—a position that did not involve commanding troops. After initially hesitating, Su eventually refused and formed an alliance with Zu against Yu. Upon hearing this, Wen, whom Yu had made the governor of Jiang Province (江州, modern Jiangxi) to defend against Tao, the governor of Jing Province (荊州, modern Hubei), wanted to quickly move to help defend the capital Jiankang, as did the local forces to the east of the capital, but Yu declined all help, wanting Wen to remain in position against Tao and believing that he can defeat Su easily. Fearful that Yu would be defeated by Su, Wen headed toward the capital anyway. Before Wen arrived, Su was able to capture the capital in early 328 and take Emperor Cheng and Empress Dowager Yu hostage. Bian died in the battle, and Yu Liang was forced to flee to Wen. Su allowed his soldiers to pillage the capital, and officials and commoners alike had their possessions—as well as clothes—stripped by Su's army, which even seized Empress Dowager Yu's servant girls. Empress Dowager Yu, humiliated by Su and fearful of what was to come, soon died in anxiety.
Su organized a new government, with Wang Dao, whom Su respected, as the titular regent, but with Su himself in actual power. Meanwhile, Yu and Wen organized efforts to recapture the capital. Wen's cousin Wen Chong () suggested inviting Tao, a capable general with a sizable army, to be the supreme commander of the army. However, Tao, still resentful of Yu, initially refused. Eventually he relented and joined Wen and Yu. They advanced east toward Jiankang. In response, Su forcibly took Emperor Cheng to the fortress of Shitou and put him and his attendants under virtual arrest. Meanwhile, Wang was secretly ordering the commanderies to the east to rise against Su, and he eventually persuaded Su's general Lu Yong () to defect with him to Wen and Tao's army as well. Chi also arrived with his forces from Guangling (廣陵, in modern Huai'an, Jiangsu).