In DGSD, Khitan Liao Emperor Yeh Lut Hung Gei had big ambitions: he was planning to invade the Northern Sung Kingdom as part of a larger campaign of conquest that would eventually include Dali Kingdom and Hsi-Hsia Kingdom.
Was the Liao Empire, at that point of its history, strong enough to undertake such an endeavor? By the time of Yeh Lut Hung Gei, the Liao Empire was already on its last legs, and within a generation, would be wiped out by the Jurchen. If the Liao Empire had attacked the Northern Sung as Yeh Lut Hung Gei planned, the Khitan likely would have been on the losing end of that war. Even when the Liao Empire was at the height of its strength, it was unable to fully conquer the Sung territories. During the reign of Yeh Lut Hung Gei, the Liao were probably not strong enough to take on the Sung.