Around or in 383, when Fu Jiān prepared a campaign to destroy Jin and unite China, Murong De and his troops were mobilized, and he left his own sons, Murong Na, and Consort Gongsun in Zhangye; before leaving, he gave his mother a golden knife as a sign of his devotion.
Consort Gongsun was spared because of her old age, while Murong Na's wife Lady Duan, who was then pregnant, was imprisoned to await for execution after she gave birth.
Consort Gongsun and Lady Duan, however, were rescued by Murong De's former subordinate Huyan Ping (呼延平), who escorted them to the Qiang tribes.
For the first several years of Later Yan's existence, he often carried out campaigns to secure areas that were being held by semi-independent warlords.
The army was forced to withdraw, and Murong Chui died on the way back to the capital Zhongshan (中山, also in modern Baoding).
On several occasions his subordinates asked him to take imperial title, but each time, after receiving confirmation that Murong Bao was still alive, he did not do so.
Meanwhile, however, Murong Lin, who had briefly claimed imperial title himself, fled to Yecheng and suggested to Murong De to abandon Yecheng, which he considered too large to defend properly, and head south of the Yellow River to Huatai (滑台, in modern Anyang, Henan).
Murong Bao had, unaware that Murong De had effectively declared independence, started a campaign against Northern Wei, but his tired soldiers mutinied, initially forcing him back to Longcheng, and then further capturing Longcheng and forcing him to flee south.
Murong De personally attacked Fu Guang and killed him, but as he did so, his nephew Murong He (慕容和) the Prince of Luyang, left to guard Huatai, was assassinated by his general Li Bian (李辯), who then offered the city to Northern Wei.
Northern Wei's viceroy Suhe Ba (素和跋) quickly entered Huatai and defeated returning armies commanded by Murong De and his cousin Murong Zhen (慕容鎮) the Prince of Guiyang, and the other cities of the Huatai region then surrendered to Northern Wei as well.
Murong De considered putting Huatai under siege, but at the advice of his general Han Fan (韓範), who argued that sieging Huatai would be difficult, he took his troops and advanced east instead, against the Jin governor of Qing Province (青州, modern central and eastern Shandong), Pilü Hun (辟閭渾).
From this point on, Murong De carried out few military campaigns, appearing to be content with controlling his small but stable empire.
Also in 403, at Murong De's permission, the official Han Zhuo (Han Fan's brother, name not in Unicode) carried out a reform of a policy that Murong De had previously established—that if people in his state had been forcibly relocated, they would be exempt from property taxes, which however led to fraudulent claims of forcible relocation—leading to restoration of proper taxation.
In 402, in the aftermaths of Huan Xuan's takeover of the Jin regency by force, several generals who unsuccessfully opposed Huan—Liu Gui (劉軌), Sima Xiuzhi (司馬休之), Gao Yazhi (高雅之), and Liu Jingxuan (劉敬宣) -- fled to Southern Yan.
Han Fan agreed with the proposal, feeling that Huan was not a good general and could be easily defeated.
Murong De was hesitant—citing that he had long wanted to recover territory from Northern Wei and never considered advancing south—and then, with a number of other generals agreeing with him, did not carry out Gao's plan.