"The Rear-Gaurd" is a an interesting poem to look at stanza by stanza because the first thing a reader should notice is that the poet did not make each stanza equal in length.
The first stanza is the poem's shortest at only three lines long. Readers are introduced to a soldier that is groping along a darkened tunnel with only his flashlight for illumination. Due to the poor visibility the soldier is forced to rely on his other senses. We see him sniffing air that has been made "unwholesome." That air could be tainted with the smell of poison gas or dead and rotting bodies.
Stanza two tells readers a little bit more about the soldier's surroundings. The tunnel system is a disaster. All kinds of materials are strewn about the place, and the stanza gives readers an uneasy feeling because many of the shapes around the soldier are "vague." We are also told that he is well below the surface. 50 feet below ground puts the soldier in a completely different world than the one above where a battle rages on.
Stanza three is by far the most harrowing of the stanzas. The soldier trips on another soldier, and our initial thought is that the man is alive. The soldier asks a question. Then he screams an insult and gives the man a "savage" kick. This is all meant to show how close to the ragged edge the protagonist is. As it turns out, the other soldier is dead. The description of this dead soldier is in the present tense, which gives an eeriness to the poem. Sassoon's style of description makes the dead man seem alive at the same time.
Terribly glaring up, whose eyes yet woreAgony dying hard of ten days before;And fists of fingers clutched a blackening wound.
Unloading hell behind him step by step.