In 427 AD, Changsu transferred his seat of government from the narrow mountain valleys along the Yalu River to the broad river basin at P'yong'yang. Both Paekche and Silla viewed the move as a serious, common threat.
King Nulji worked to free himself from Goguryeo domination. He set up diplomatic relations with Goguryeo on an equal footing in 424, and established a military alliance (Naje Dongmaeng) with Baekje in 433 to help counter the Goguryeo threat.
Silla's King Nulchi concluded an alliance with Paekche's King Biyu, in an attempt to counter the implications of that move and to check the pressure being exerted by Koguryo on their northern frontiers, by exchanging ambassadors in 433 and 434. Although Silla was a protectorate of Goguryeo at this time, Silla and Baekje now allied themselves against Goguryeo.
Despite several occasions during the ensuing years when the two kingdoms carried out joint military operations against Koguryo, the Silla-Paekche alliance proved to be an exercise in futility:
Facts and Figures :
* In 475 AD, Koguryo seized the Paekche capitol at Hansong near modern Kwangju just below Seoul, captured King Kaero and beheaded him.
In the Goguryeo assault of 475, Crown prince King Munju went to Silla to request help. According to the Samguk Sagi, he returned with 10,000 Silla warriors but was too late to prevent the fall of the capital.
Barely managing to preserve its national existence, Paekche moved its capitol further south to Ungjin near modern Kongju.
* King Dongseong of Paekche established an alliance with Silla through his marriage of a Silla noblewoman in 493, and the two countries united in attacking Goguryeo in 495.
* After King Song restructured Paekche internally and increased its military strength, he turned his attention to the recovery of Paekche's former territory in the Han River basin. He made a pact with Silla's King Chinhung and together they struck northward to take advantage of growing internal dissension in Koguryo. The combined forces of Silla and Paekche surged against Koguryo in 551 AD, sweeping across the fertile valleys of central Korea. Paekche's King Song-myong had only a brief time in which to celebrate his victory over Koguryo along the lower reaches of the Han River. Flushed with the success of his own armies, Silla's King Chinhung ordered his troops westward and drove Paekche's forces completely out of the Han-Imjin River basins, securing the entire Han River basin and the Bay of Namyang for Silla.
Silla warriors drove through the Han and Imjin River valleys, taking possession of the richest agricultural land in the peninsula, the military and draft labor services of peasants, iron mines, and ten communities along the upper reaches of the Han River. The campaign greatly increased Silla's wealth and opened an easier route to China from ports on the Yellow Sea. The success of this venture foretold of even greater expansion by the Kingdom of Silla.
The conquest gave Silla control over a large settled population in a rich agricultural area that provided additional taxes, manual labor, and military service. It also opened a gateway to communicate directly with China across the Yellow Sea and brought technological gains to Silla such as an existing iron industry. Silla's unexpected seizure of the Han River basin came as a devastating blow to King Song-myong who saw his dreams end in failure.
Enraged by Silla King Chinhung's treachery, King Song-myong turned against his former ally and personally launched a frontal assault against Silla across the Kum River in 554 AD. King Song-myong died in the ensuing campaign, killed in the confusion of a deadly night battle at Kwansan Fortress near modern Okch'on, a battle that severed a Silla-Paekche alliance that had endured for 121 years.
Beginning with the Silla victories in the Han-Imjin River basin, Paekche saw Silla as a mortal enemy. Paekche turned to Koguryo for help (YoJe Dongmaeng) and launched continuous attacks against Silla, a confrontation that presaged a great storm that soon erupted over northeast Asia.