Since Emperor Min of Jin ascended the throne, the situation of the Western Jin Dynasty deteriorated with each passing day. The city of Chang'an was almost annually invaded by the forces of the Han general Liu Yao. Although Liu Yao was repelled each time, the conditions in Chang'an gradually worsened. Shi Le had already occupied most of the Guandong region, while Li Xiong's Cheng Kingdom, based in the Hanzhong area, entrenched itself south of Chang'an. The areas near Luoyang, farther from Chang'an, and southern Shanxi had already fallen under the control of the Han state, and Sima Rui, the Prince of Langya in the distant Jiangnan region, also possessed considerable strength.
During the crisis-ridden situation of the Western Jin Dynasty, Sima Rui, the Prince of Langya, began to scheme for his own benefit. He was well aware that Chang'an was in imminent danger and would inevitably fall, and once the city was captured, he would have a great opportunity to ascend the imperial throne. Therefore, he had no intention of sending troops to rescue Chang'an. Although some Jin loyalists in the Jiangnan region, like Zu Ti, advocated for a northern expedition, Sima Rui only offered them empty promises, leaving these people helpless and frustrated.
In 316 AD, Emperor Min of Jin was captured, ending the 36-year Western Jin Dynasty.
Before Emperor Min of Jin was captured, he wrote a secret edict appointing Sima Rui as his successor to the throne. Song Zhe, the governor of Hongnong, hid the edict on his person and secretly fled Chang'an. After a long and arduous journey, he finally arrived in Jiankang (present-day Nanjing, Jiangsu) and met with the Prince of Langya, Sima Rui.
Sima Rui's grandfather, Sima Zhou, was a son of Sima Yi by a concubine and was enfeoffed as the Marquis of Nan'an during the reign of Cao Fang. Later, Sima Yi launched the Gaopingling Coup, successfully seizing real power over the Cao Wei government. To eliminate Wei and prepare thoroughly for ascending the throne as emperor, Sima Yi appointed several of his sons as military governors of key regions. Sima Yi held Sima Zhou in high regard, entrusting him with the defense of Ye, a crucial garrison and supply depot during the Cao Wei period. After the founding of the Western Jin Dynasty, Sima Zhou served successively as Right Vice Director of the Department of State Affairs, General Who Pacifies the Army, Grand General Who Pacifies the East, Holder of the Imperial Emblem, and Commander of Military Affairs in Xuzhou, among other posts. He was later enfeoffed as the Prince of Dongguan and subsequently as the Prince of Langya. After Sima Zhou's death, his eldest son, Sima Jin, inherited the title of Prince of Langya. Sima Jin was Sima Rui's father. Upon Sima Jin's death, Sima Rui inherited the title of Prince of Langya.
After reading the secret edict, Sima Rui was overjoyed and immediately prepared to ascend the throne. He put on the imperial crown and dragon robe, then sat on the dragon throne. All the civil and military officials knelt down and shouted "Long live the Emperor" three times. Sima Rui announced that after his enthronement, the dynasty would still be called Jin, with its capital at Jiankang and the reign title Daxing. This year was 318 AD, also known as the first year of Daxing, and Sima Rui was honored as Emperor Yuan of Jin. Historically, this regime is referred to as the Eastern Jin, to distinguish it from the already fallen Western Jin.
During the establishment and consolidation of the Eastern Jin regime, it received support from various quarters, first from local landlords and powerful families in Jiangnan.
Although the power of these local landlords and magnates was weaker than that of the northern aristocratic families who had migrated south, they still held very significant influence in their regions. Among these local magnates, the Zhou family enjoyed the greatest social prestige. Zhou Li's household produced five marquises, and Zhou Qi pacified the Jiangnan region three times; these two brothers were the leaders of the Jiangnan aristocracy. Additionally, Lu Ye, Ji Zhan, Gu Rong, He Xun, Hua Tan, and others were also important aristocratic families in Jiangnan. Most of them served as civil officials and played a very crucial role in the early establishment of the Eastern Jin Dynasty.
Border generals such as Liu Kun, Duan Pidi, Zhang Gui, Murong Wei, Shao Xu, and Chen An played a crucial role in the establishment of the Eastern Jin regime. Although most of them died in battle, their unwavering loyalty and courage were truly admirable. Without their arduous campaigns in the north, which blocked the northern ethnic forces from launching attacks on the Jiangnan region, the Eastern Jin would never have been able to establish its rule smoothly.
During the process of consolidating the Eastern Jin regime, the migrants also played a very important role. Due to the impact of the wars in the Central Plains, many homeless northern people fled to the south. The Eastern Jin government accepted these displaced people and established many migrant commanderies and counties in the Jianghuai region to settle them. These migrants served as a protective barrier for the Eastern Jin regime. Later, they further integrated with the local people in Jiangnan, strengthening economic, cultural, and social exchanges between the north and south.