燕雲十六州
When the Song were successful in finally incorporating the Northern Han in 979, the Song decided to launch an offensive against the Liao in the Sixteen Prefectures. Emperor Taizong of Song led his weary and ill-supplied troops from toward the Liao Southern Capital (present-day Beijing). The Liao boundary was reached in May and they initially encountered little resistance. By July 20, they had attacked the Southern Capital. Ten days later, the first contingent of Liao cavalry arrived. The ensuing Battle of Gaoliang River on August 1 near the Southern Capital resulted in a complete rout of Song forces, who had to retreat back to Kaifeng. The Sixteen Prefectures would remain in Liao hands.