Sima Yan’s Jin dynasty briefly reunified Three Kingdoms China
From 266 to 420 CE the Jin Empire, founded by Sima Yan—Emperor Wu—succeeded Cao Wei. Western Jin (266–316) took Eastern Wu in 280 and ended the Three Kingdoms split, then tore itself apart in the War of the Eight Princes before northern collapse forced an eastern refuge.
The Sima family climbed to power inside Cao Wei, the strongest of the Three Kingdoms. Sima Yi, its regent, staged the Incident at the Gaoping Tombs in 249 and after that his clan eclipsed the ruling Cao. His son Sima Zhao conquered Shu Han in 263 and was made King of Jin, a title drawn from the ancient Zhou-era state where the family had its roots. Zhao's son Sima Yan declared himself emperor on 4 February 266, forced the last Wei ruler to abdicate, and let him live out his days honourably as Prince of Chenliu.
Reunification in 280 brought a decade of peace and conspicuous aristocratic luxury. It also carried the seeds of collapse. Emperor Wu insisted that his eldest son, Sima Zhong, succeed him despite an apparent developmental disability, and he handed princes military and civil power to shield the heir. When Wu died in 290, Empress Jia ruled from behind her husband, corruption spread, and after her overthrow in 300 the princes fought one another for the regency.
In 304 the Cheng-Han and Han-Zhao states broke away, and famine and revolt ate into the north. Han-Zhao troops under Liu Cong sacked Luoyang in 311 and captured Emperor Huai, who was later executed; Chang'an fell soon after. In 318 Sima Rui was enthroned at Jiankang, modern Nanjing, opening the Eastern Jin.
In the south, emperors were weak and great immigrant clans such as the Wang of Langya and the Xie of Chenliu held real power, a popular saying claiming that Wang Dao and Sima Rui ruled together. Northern refugees got special commanderies and registers, and the south eventually outgrew the north in population. At the Fei River in 383 a heavily outnumbered Jin army shattered Former Qin, yet court intrigue undermined campaigns like Huan Wen's. Huan Xuan briefly usurped the throne in 403, and the general Liu Yu restored and then dominated the dynasty.
Source: Jin dynasty (266–420)
