Drunk with fatigue; deaf even to the hoots
Of tired, outstripped Five-Nines that dropped behind.
Gas! GAS! Quick, boys! – An ecstasy of fumbling,
Fitting the clumsy helmets just in time;
But someone still was yelling out and stumbling,
And flound'ring like a man in fire or lime . . .
Dim, through the misty panes and thick green light,
As under a green sea, I saw him drowning.
In all my dreams, before my helpless sight,
He plunges at me, guttering, choking, drowning.
If in some smothering dreams you too could pace
Behind the wagon that we flung him in,
And watch the white eyes writhing in his face,
His hanging face, like a devil's sick of sin;
If you could hear, at every jolt, the blood
Come gargling from the froth-corrupted lungs,
Note the  alliterative ,  assonant   “w” and “i” sounds in “watch the white eyes writhing”.  The effect is like a tongue twister, difficult to say, a reflection of pain and words that can’t describe the horror. 
 The phrase “white eyes” may also refer to the blindness of those who promote war, the horror of which can only be understood by those who experienced it.