Gareth,

It's from Sassoon's "Suicide in the trenches" :

SUICIDE IN THE TRENCHES
By Siegfried Sassoon

I knew a simple soldier boy
Who grinned at life in empty joy,
Slept soundly through the lonesome dark,
And whistled early with the lark.

In winter trenches, cowed and glum,
With crumps and lice and lack of rum,
He put a bullet through his brain.
No one spoke of him again.

You smug-faced crowds with kindling eye
Who cheer when soldier lads march by,
Sneak home and pray you'll never know
The hell where youth and laughter go.

Walrus