In 238, Dongcheon was able to ally with the Wei, one of the three Chinese kingdoms in the northwestern area, in order to
destroy the Gongsun family and erase its influence over
Liaodong Peninsula and other areas bordering Goguryeo. The war on the Gongsun was a victory, but Goguryeo's ally, Wei, eventually became a new threat. Dongcheon was forced to flee the capital. The exact location of the new capital is still disputed. King Dongcheon received news of Yu Yu's death and ordered that a memorial be made for Yu Yu the Patriot. Then, he led his armies in the attack to push the Wei forces out of Goguryeo territory. General
Mil U () and
Yu Okgu () also repulsed the Wei forces. The Goguryeo forces won this battle, and regained all of the territory that had been lost from defeats against the Wei. This passage was not paralleled in Chinese records, and Hiroshi Ikeuchi points out its errors: the author of this passage in
Samguk sagi regarded the region of South Okjeo and Lelang as identical, while in fact they are on opposite sides of the peninsula; also, the references to the "Eastern Department" for Yu Yu and Mil U are
anachronistic, since Goguryeo did not divide the country into departments until the middle of the Goguryeo dynasty — that is, after Dongcheon's reign. As such, Ikeuchi considered the
Samguk sagi's stories to be highly fictional. In 243, he named his son
Yeonbul the crown prince and successor to the throne. He attacked
Silla, another of the Three Kingdoms to its south, in 245 but made peace in 248. The records are found in Samguk sagi under the annal of Isageum (Silla's ruler) that Dongcheon invaded the northern area of Silla, but the validity of a peace agreement has not been fully explained, since given that Goguryeo was under harsh attacks from the northern area, an invasion of Goguryeo into Silla would be logically incomprehensible. It is well accepted that this invasion indicated a sudden inflow of refugees from Goguryeo into a bordering area with Silla. ==Death and succession==