Analyse the poem in detail, i love this poem with all my heart it's beautiful.
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.

its beautiful indeed and sad at the same time x_x

this poem reflects light upon the young boys (soldiers) who come to take their own life (in this case so..) under the great pressure of their work on them which itself puts their life on stake,

the poet tells that these boys were once as happy to serve as we are at the soldier marches, ready to take up any trouble what he faces for the sake of the nation ,

its just this easy at the start, but we will never the know the pain and responsibility they go through in the hell (battlefield) where these once laughing young youth goes, where they might not even end up not seeing the light of another day.

hope this answer helps

:)

i think the answer would mist likely be an adjective clause, because unhappy is an adjective.