You will do the same to a German
Soon, no doubt, if it be your pleasure
To cross the sleeping green between.
It seems you inwardly grin as you pass
Strong eyes, fine limbs, haughty athletes,
Less chanced than you for life,
Bonds to the whims of murder,
Sprawled in the bowels of the earth,
The torn fields of France.
What do you see in our eyes
At the shrieking iron and flame
Hurled through still heavens?
What quaver—what heart aghast?
Poppies whose roots are in man's veins
Drop, and are ever dropping;
But mine in my ear is safe—
The second rhetorical question relates to the terrible fears of the young men. Here, ‘quaver’ refers to trembling and terror, rather than the short musical note. 
 The ‘heart aghast’ brings this section to a climax. The two words are  assonant  with elongated vowels. It also picks up the  alliterative  ‘h’s of the previous line, as if mimicking gasps of fear.
Break of Day in the Trenches