Running down a short list of poetic and rhetorical techniques, I find in this poem instances of alliteration (fade, figures, file, foul, flood), enjambment (coming / in file), hypallage (quaking), metaphor (steel wings), rhyme (pass, morass, grass), and rhythm. With a longer list I am sure that you could find more.
Let me take the last of these, rhythm, in a bit more detail. The first step in analyzing rhythm is scansion, the usual approach in English being to mark the stressed (/) and unstressed (x) syllables. Then the lines can be divided into feet. Not everyone agrees that feet are meaningful in English prosody, but it makes it easier to write about the rhythmic variation, since there’s standard terminology for writing about feet.
x x / x / x /
Will they nev- | er fade | or pass!
x / x x / x / x / x x / x
The mud, | and the mis- | ty fig- | ures end- | lessly com- | ing
x / x x / x /
In file | through the foul | morass,
x x / / x x / x x / x /
And the grey | flood-wat- | er rip- | ping the reeds | and grass,
x x / / / x
And the steel | wings drum- | ming.
The general rhythm is iambic, but there are frequent substitutions with anapests (through the foul), and occasional substitutions with trochees (flood-wat-) and spondees (wings drum-). The frequent rhythmic substitutions could suggest the soldiers stumbling in the difficult terrain, or limping with injuries.
There is a spondee in the last line of each stanza, resulting in three consecutive stressed syllables. In the first stanza, these slow beats could suggest the explosions of the artillery barrage (the ‘steel wings drumming’). In the second, perhaps they indicate the farmer’s difficulty in articulating his experiences to the ‘old folks’.
The second and fifth lines of each stanza have ‘hypercataletic’ or ‘feminine’ endings (that is, with an extra syllable following the last complete foot). These trailing endings could suggest the figures disappearing into mist in the first stanza, the farmer’s aporia in the second, and the river flowing away in the third.