Under his father's reign, he held the office of pajinchan, who apparently was responsible for maritime affairs, and played a key role in developing the country's diplomatic links with Tang China.

After the small isolated pockets of resistance were eliminated, Munmu was the first ruler ever to see the Korean peninsula completely unified.

After the fall of Goguryeo, Tang created the Protectorate General to Pacify the East and attempted to place the entire Korean peninsula, including Silla, under its rule.

To prevent this, Munmu forged alliances with Goguryeo resistance leaders such as Geom Mojam and Anseung, and launched a frontal attack on the Tang forces occupying former Baekje territories.

However, King Munmu formally apologized and offered tribute, and Emperor Gaozong ordered a withdraw and recalled Kim In-mun.

In 675, Li Jinxing (Hanja:李謹行) reached Silla territory with Mohe forces that submitted to Tang.

This victory, and the maintenance of Silla's independence, is generally regarded as a critical turning point in Korean history.

King Sinmun awoke from the dream, rode out to the sea and received the bamboo flute named Manpa-sikjeok (萬波息笛, 만파식적).