After the move of the capital, the Baekje power structure fell into chaos when the traditional Buyeo-descended aristocracy clashed with the local Mahan-based clans; rivalry within the old aristocracy, suppressed by previous powerful kings, resurfaced.
He sought to refortify Baekje’s remaining defenses and strengthen the country's position against Goguryeo. He was successful in 476 in gaining suzerainty over the kingdom of Tamna on Jeju island.
In the midst of this instability, the chief general Hae Gu took control of the military, and, after the death of Munju's son (Samgeun's younger brother) Gonji in 477, exercised effective rule over the country.
That year, Munju was murdered by an agent of Hae Gu