They can hear shots being fired in the distance. The line of men keep going and going. The battle now changes to an overhead view of the battle. You can see the dusty and sweaty soldiers marching under the sun. The soldiers take cover in the holes in the ground.
Artillery shells explode, wagon wheels turn, and horses sweat.
The people continue to walk. In a since what the author is seeing is a battlefield with men dying and men running. The scene is disorderly and chaotic with the stench of death everywhere. I think this poem symbolizes a person dyeing. When the men are walking toward the fighting it is like a person slowly going unconscious.