In the first of four stanzas, Owen presents the death-like calm before the storm of the gas attack. Alliteration and onomatopoeia join with powerful figurative and literal images of war to produce a pitiful sense of despair. "Bent beggars", "knock-kneed", cough and "curse" like "hags" through "sludge." All of this compressed into just two lines! The third line places the speaker of the poem with this trudging group. In the simple "Men marched asleep" sentence, the three beats imitate the falling rhythm of these exhausted men. The pun "blood-shod" makes its grim effect on us slowly. We guess, too, that "blind" and "lame" suggest several levels of debilitation. The stanza ends with the ironic-quiet sounds of the "shells" dropping "softly behind."
In contrast to the first stanza, the second stanza is full of action. The oxymoron,"ecstasy of fumbling", seems at first odd, but then perfect, as a way to describe the controlled panic -instantly awakened with heightened sensibility- of men with just seconds to find a gas mask. "But..." tells all. One man is too late and is seen only through the "green sea" of mustard gas, "yelling... stumbling...drowning...guttering...choking."
The third stanza's brief two lines emphasize the nightmare these events continue to be for our speaker.
In the last stanza, Owen becomes more insistent as he drives atus with the steady rhythmic beat of iambic pentameter. We feel the "jolt" of the wagon, see the "white eyes writhing" in this "hanging face," and, most horribly, hear the "gargling "of the blood...