There are several figures of speech the first two stanzas.
Helmet and rifle, pack and overcoat
Marched through a forest. Somewhere up ahead
Guns thudded. Like the circle of a throat
The night on every side was turning red.
The first is the personification of helmet, rifle, pack and overcoat marching. This takes away the identity of the soldiers and creates an image that rather than men, they are simply equipment for killing and protection. The second is the simile "like the circle of a throat" the night is lighting up red (presumably with bombs). In this case I think the image of the throat is to signify choking. It is as if the night and bombs are closing in on these marching soldiers and their lives will be choked away.
They halted and they dug. They sank like moles
into the clammy earth between the trees.
And soon the sentries, standing in their holes,
Felt the first snow. Their feet began to freeze.
The final figure of speech is the simile "sank like moles." Moles burrow into the ground for protection. Moles are also blind. This image suggests the trenches built in WW1 for protection left the men inside them blind to whatever was going on outside them.
These figures of speech reinforce the mood of fear and helplessness presented in the rest of the poem. There is a pervasive tone of impending death (as in most WW1 literature). The men in the poem (as the real soldiers, historically) do not think nor feel like men at all. Instead, they are robots and machines, working simply to survive or to await inevitable death.