Having brought Emperor Yuan (Sima Rui) to submission with his military force, Wang Dun had paramount authority.
He served as an assistant to Emperor Hui's crown prince Sima Yu, and when Sima Yu was falsely accused of crime by Empress Jia Nanfeng and deposed in February 300, he risked his life to attend the crown prince's farewell, and received renown from that.
Later, as Jiang (江州, modern Jiangxi and eastern Hubei) and Jing (荊州, modern Hubei and Hunan) Provinces became overrun by agrarian rebels, the strongest of whom was Du Tao (杜弢), the leader of Yi Province (modern Sichuan and Chongqing) refugees who had fled from Cheng-Han, Sima Rui put Wang Dun in charge of the western province operations, and Wang's effective coordination allowed the rebels to be eventually suppressed.
Wang Dun was further encouraged by his assistants Qian Feng (錢鳳) and Shen Chong (沈充), both of whom persuaded him to plan a military confrontation with Emperor Yuan.
In 320, however, when Emperor Yuan, against Wang's request, made Sima Cheng (司馬承) the Prince of Qiao the governor of Xiang Province (湘州, modern Hunan) instead of Shen, Wang Dun was not yet ready to fully break with Emperor Yuan, and therefore allowed Sima Cheng to take his post.
Wang Dun, satisfied, allowed Emperor Yuan to remain on the throne (Although he toyed with the idea of removing Sima Shao the Crown Prince, fearful of Crown Prince Shao's decisiveness and diligence, Wang ended up not carrying out the idea), and personally withdrew back to his home base of Wuchang (武昌, in modern Ezhou, Hubei).
However, he did not know that Wen had actually been working with Emperor Ming's brother-in-law Yu Liang, and once Wen arrived in Jiankang, he revealed Wang Dun's illness and his plan, and Emperor Ming decided to take preemptive action, declaring Wang Dun a renegade and summoning the generals on the northern border to come to his aid.