直皖戰爭
As chief of staff, Guo trained the younger Zhang's brigade into one of the best brigades in the Fengtian army. In July of that year, the Zhili–Anhui War erupted, and Guo was appointed by Zhang Zuolin as commander of an allied force with the Zhili clique against the Anhui clique. Guo's forces decisively beat the Anhui forces in Tianjin, and he grew more and more in Zhang Zuolin's confidence.