The main message of this poem is that it is not "sweet and fitting to die for one's country" as so many people choose to believe; war is tragic and awful and gruesome and miserable, and so are the effects that it has on young people. Owen uses a simile when he says that the soldiers are "Bent double, like old beggars under sacks" and another simile when he says that they are "coughing like hags." A metaphor compares the effects of their extreme fatigue to being "Drunk." All three of these comparisons paint pictures of the awful effects war has on these young, healthy men. They now seem old, decrepit, and weak, when they ought to be vital, vibrant, and innocent.
Owen creates an auditory image with his description of the "hoots / Of gas-shells dropping"; the soldiers are so tired that they do not even hear these sounds, but the onomatopoetic word, hoots, helps us to hear them. Its a rather eerie sound. Owen creates a visual image with his description of the speaker's sight, "Dim through the misty panes and thick green light, / As under a green sea, I saw him drowning." He watches a fellow soldier who does not put on his gas mask in time and who inhales the chlorine gas that will burn the tissue in his lungs and, eventually kill him. Another auditory image describes the "gargling [coming] from the [soldier's] froth-corrupted lungs," as he dies an agonizing and painful and prolonged death. All three of these images create a frightening mood of confusion and pain; they make the reader feel the dread experienced by the soldiers, and this helps to prove the poet's point.