燕雲十六州
Because the land routes between the Song and Jin were controlled by the Liao, diplomatic exchanges had to occur by traveling across the Bohai Sea. Negotiations for an alliance began secretly under the pretense that the Song wanted to acquire horses from the Khitans. Song diplomats traveled to the Jin court to meet Aguda in 1118, while Jurchen envoys arrived in the Song capital Kaifeng the next year. At the beginning, the two sides agreed to keep whatever Liao territory they would seize in combat. In 1120, Aguda agreed to cede the Sixteen Prefectures to the Song in exchange for transfer to the Jin of the annual tributary payments that the Song had been giving the Liao. By the end of 1120, however, the Jurchens had seized the Liao Supreme Capital, and offered the Song only parts of the Sixteen Prefectures. Among other things, Jin would keep the Liao Western Capital of Datong at the western end of the Sixteen Prefectures. The two sides agreed that the Jin would now attack the Liao Central Capital, whereas the Song would seize the Liao Southern Capital, Yanjing (modern-day Beijing).