"…deaf even to the hoots Of tired, outstripped Five-Nines that dropped behind" (7-8).
"Five-Nines" are gas shells, the dropping of which starts off the action of the rest of the poem. The fact that even the shells seem "tired" and "outstripped" suggests that the war might be dragging on too long.
"Gas! Gas! Quick, boys! – An ecstasy of fumbling, Fitting the clumsy helmets just in time;" (9-10)
The exclamations at the beginning of line 9 speed up the pace of the poem, bringing us into the action with all the drama that the soldiers themselves experience. The hyphen in the middle of the line reinforces this urgency, moving through the pause in the middle of the line as if it, too, is suddenly sped up.
Men marched asleep. (5)
War seems like a continual process in this line: even when the men are "sleeping," they're advancing or retreating from the field of battle. The image as a whole contributes to the ghost-like quality of the soldiers in this poem.