We march West once more is an unofficial name for a verse in Mthethwa that is mentioned multiple times in the story. Before the Fifteenth Legion sets off for Callow, Nauk quotes the first line, and the rest of the group replies with the second. It's a well-known verse; Catherine describes it as 'famous'. The last two lines are left off, both times the first lines are spoken.
It is unclear if this is a verse from a song or a poem, and whether there are more verses or not.
We march West, once more
Waging that same old war
Onward to the fields of Callow
Swift death and graves shallow