Sisters would wish girls too could shoot, charge, curse, . . .
Brothers—would send his favourite cigarette,
Each week, month after month, they wrote the same,
Thinking him sheltered in some Y.M. Hut,
Where once an hour a bullet missed its aim
And misses teased the hunger of his brain.
His eyes grew old with wincing, and his hand
Reckless with ague.  Courage leaked, as sand
From the best sandbags after years of rain.
But never leave, wound, fever, trench-foot, shock,
Untrapped the wretch.  And death seemed still withheld
For torture of lying machinally shelled,
At the pleasure of this world's Powers who'd run amok.
He'd seen men shoot their hands, on night patrol,
Their people never knew.  Yet they were vile.
"Death sooner than dishonour, that's the style!"
This refers to those who conducted the War so ineptly. ‘Powers’ could also refer to the authoritarian  attitudes which wear the soldier down so profoundly that he is unable to cope. 
 Note also that this is  a New Testament  biblical reference, St Paul to Ephesians 6:12, that Owen’s original readers would have recognised. 
 1> For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms.