During his reign, the Three Kingdoms were at war with each other, as alliances shifted and the neighboring China also experienced a change of dynasties. Because reliable historical sources are hard to find for the Three Kingdoms period, the specifics of Mu's policies are not known.
Reign
Early in his reign, Mu attacked Silla several times. He also requested assistance from Sui dynasty of China to attack Goguryeo. Following the Goguryeo–Sui War, the Sui was replaced by the Tang dynasty in China in 618. In 627, he attempted to recover land lost to Silla, but stopped when Tang intervened diplomatically. The same year, he sent the Buddhist monkGwalleuk to Japan with texts on Buddhism, astronomy, history, and geography. He formally established the Mireuksa temple in 602. He is also said to have ordered the repair of Baekje's Sabi Palace in 630, and the construction near his palace of the earliest known artificial lake in Korea. His policies in the latter half of his reign, which emphasized construction projects at the expense of national defence, are often thought to have contributed to the fall of Baekje which took place twenty years after his death. There is reason to believe that he moved the capital of Baekje from Sabi in Buyeo County to Iksan, at least briefly. Archaeological evidence in Iksan, including tombs attributed to Mu and his wife Queen Seonhwa, appears to confirm this. Mu retained close ties with Tang China, but Tang later allied with Silla in the wars that ultimately unified the Korean Peninsula under Silla's rule by 668.
Seodong-yo
The Samguk Yusa relates a legend regarding Mu's marriage to a princess of Silla, although historians consider it unlikely to be true, given the hostilities between the rival kingdoms. In this story, the young Seodong falls in love with Silla princess Seonhwa, and intentionally spreads a song about the princess and himself among the people. Thanks to this song, King Jinpyeong of Silla banishes the princess, and Mu marries her and becomes the king of Baekje.
Family
Father: Wideok of Baekje
Mother: Yeon Gamo
* Brother: Jin'ni-Ō – settled in Japan and became ancestor of the Ōuchi clan and Toyota clan.
* Queen: Lady Sataek – daughter of Minister Sataek Jeokdeok, later became the Empress Dowager Munjeong during her son's reign.
** 31st king, Uija of Baekje – last king of Baekje, known as Buyeo Uija before he became king.
* Queen: Princess Seonhwa – daughter of King Jinpyeong of Silla.
