In the preface, Yan Geling explained the time, events and reasons of writing. It provides a big environmental background for the whole poem.
Yan song combination generally refers to Yan songs. Ge Yanxing is a poem by Gao Shi, a poet in the Tang Dynasty. This poem sums up the general frontier wars, mainly exposing the lords' arrogance, underestimating the enemy and not caring for the soldiers, which led to the failure of the war.