Answer:
The Song Dynasty (960-1279) was a period of significant military innovation and development in China. The Song Dynasty inherited military institutions designed for conquest and transformed them into the tools of an established empire. The Song Dynasty used military force to quell the Liao dynasty and to recapture the Sixteen Prefectures, a territory under Khitan control since 938 that was traditionally considered to be part of China proper. The army of the Song Dynasty comprised mainly professionals, and the best troops were stationed around the capital under leaders directly controlled by the emperor. Contingents from the palace troops were rotated out to frontier defense units for three-year tours.