pursue a routed army
Chasing the north is a Chinese idiom. The Pinyin is Zhu ī B ē nzh ú B ě I, which means to pursue the defeated enemy. It comes from the Han Dynasty Jia Yi's Guo Qin Lun.
The origin of Idioms
Han Jiayi's on crossing the Qin Dynasty: "chasing the subjugated and chasing the north, one million corpses." Li Ling's answer to Su Wu Shu in the Han Dynasty: "however, you still have to cut the flag of the general, chase the north, wipe out the trace and sweep away the dust, and kill the commander."
Idiom usage
To pursue a defeated enemy
Examples
His elite horse team, also from the desert, chasing north, is his strong point. Gao Yang's "jade seat and Pearl curtain" Volume I
pursue a routed army
leaving evil unchecked spells ruin - yǎng yōng chéng huàn