An Qingxu
When the general An Lushan rose against the rule of Emperor Gaozong's grandson Emperor Xuanzong in 755, Xue Song served An's rebel Yan. The first solid historical reference to him was in 758, when An Lushan's son and successor An Qingxu was besieged by Tang forces in Yecheng and forced to send Xue to seek out aid from his father's old subordinate Shi Siming. Shi eventually did defeat Tang forces and lift the siege at Yecheng, but then killed An Qingxu himself and took over as the emperor of Yan, and it appeared that Xue continued to serve under Shi Siming at that point.