King Muyeol of Silla sought help from Emperor Gaozong of Tang, who launched the invasion of Baekje in 660.

The Silla kingdom had formed a military alliance with the Tang empire under Emperor Gaozong's reign.

[1] In 650, Emperor Gaozong received a poem, written by Queen Seondeok, from the princely emissary Kim Chunchu (who would later accede the Silla throne as King Muyeol).

Even though Baekje was allied with Goguryeo, the Han River valley separated the two states and was a hindrance in coming to each other's aid in time of war.

[1][4] The army led by Admiral Su Dingfang sailed across the Yellow Sea towards and landed on the western coastline of Baekje.

[1] The territory of Baekje was annexed by the Tang, which established five military administrations to control the region instead of Silla, which they painfully accepted.

[8] In a final effort, General Gwisil Boksin led the Baekje revival movement against Tang occupation.

[9] The Tang fleet, comprising 170 ships, advanced towards Chuyu and encircled the city at Baekgang River.