At the time of his death, he left the state under the heel of Wang Dun, but the Eastern Jin dynasty as it became known lasted until its fall in July 420, contending with the Sixteen Kingdoms in the north and occasionally in the southwest.

As the struggle to become Emperor Hui's regent (known as the War of the Eight Princes) grew in intensity, Sima Rui was humble in his dealings, and kept himself aloof from political developments in order to protect himself.

He decided to flee back to his principality, Langxie (roughly modern Weifang, Shandong), under counsel of Sima Yue's assistant Wang Dao, whom he befriended during the campaign.

His own guard Song Dian (Chinese: 宋典; pinyin: Sòng diǎn[11]) then arrived and shoved him under the ruse that they were both construction workers.

After Sima Rui got to Luoyang, he took his mother, Princess Dowager Xiahou, and headed to Langxie where they spent the next few years away from the War of the Eight Princes.

Under the advice of his wife Princess Pei, he commissioned Sima Rui as the military commander of parts of Yang Province (Chinese: 揚州; pinyin: Yángzhōu, modern Zhejiang and southern Jiangsu and Anhui) south of the Yangtze River, with his post at Jianye.

Many of the southern gentry clans initially backed Chen Min, but towards the end, they became discontented with his rule and returned to Jin by overthrowing him.

The deadlock was finally broken in February 311, when Zhou Fu, under fire by the imperial court for suggesting that they move to his base in Shouchun, was forced into rebellion.

Sima Rui, now with grounds to attack Zhou Fu, ordered his forces to march north of the Yangtze and annexed the rest of Yang province.

In July 311, not long after Sima Yue died in April, Luoyang fell to Han forces and Emperor Huai was captured during the Disaster of Yongjia.

Through Xun Fan's assent, Sima Rui began to exercise more imperial power as he was acclaimed the leader of the Jin alliance and granted special authority to appoint and dismiss chief officials.

With many officials fleeing south of the Yangtze, Wang Dao advised Sima Rui to scout for and recruit the talented men the northern emigres families to serve in his administration.

South of the Huai river and to the west of Sima Rui, the Inspector of Jiang province (Chinese: 江州; pinyin: Jīangzhōu, modern Jiangxi), Hua Yi and the Inspector of Yu province (Chinese: 豫州; pinyin: Yùzhōu, modern central Anhui at this time), Pei Xian both outrightly refused to acknowledge Sima Rui's new authority.

In the central Yangtze region, the two provinces of Jing (Chinese: 荊州; pinyin: Jīngzhōu, modern Hubei) and Xiang (Chinese: 湘州; pinyin: Xiāng zhōu, modern Hunan) had long been struggling with a refugee crisis, due to many people fleeing from the ongoing war with the Cheng-Han dynasty in Sichuan.

Sima Rui took the opportunity to further expand his influence into the central Yangtze, installing his own Inspector of Xiang and convincing Wang Cheng to give up his office.

Under the command of Wang Dun and other generals such as Tao Kan and Zhou Fang, the agrarian rebels resisting Jin rule in Jing and Xiang were gradually subjugated.

The hardliners rallied around Zhou Qi and his family to oust the northern emigres and force Sima Rui to restore the southern clans to power.

In 319, Duan Pidi's forces fell to Shi Le—who had by that point declared independence from Former Zhao as declared by Liu Yao, establishing Later Zhao in the same year—and Duan fled to another governor still loyal to Jin, Shao Xu the governor of Ji Province (Chinese: 冀州; pinyin: Jìzhōu, in what is now central Hebei).

Failing to rescue himself, he presumably faced resistance by remaining generals under him Yang Tao (楊韜) and Chen An (which had by this point defected to Han-Zhao, but bearing some loyalty to him) and was soon apparently murdered by his generals Zhang Chun (Chinese: 張春; pinyin: Zhāng chūn) and Yang Ci (Chinese: 楊次; pinyin: Yáng cì), replacing him with his relative Sima Zhan (since Sima Bao had no sons).

[21] Later in 320, Zhang Shi was assassinated by his guards Yan She (閻涉) and Zhao Ang (趙卬), acting from the rumors spread by the magician Liu Hong (劉弘).

Wang Dun, satisfied, allowed Emperor Yuan to remain on the throne, and personally withdrew back to his home base of Wuchang.