What does the setting tell us about the purpose of the text?In the poem "Hero" by Siegfried Sassoon?

Typically in poetry we use the word
scene rather than setting.  Because poems are
often much shorter than stories and leave out many details, it is often only possible to
analyze a line or a stanza based on the internal or
external scene that is depicted.

In
this poem, there are two obvious external and
internal scenes.  The first is Jack's mother's house, where she
receives the news that her son is dead.  The Officer who brings the Colonel's letter
consoles her in her time of grief with "gallant lies" about her son's heroism before he
died.  Upon hearing this, her eyes shine with a "gentle triumph" and even reflect a bit
of "joy."  The internal scenes (depicted by feelings or emotions)
at the mother's house are two-fold: the officer is lying (he believed Jack to be a
"cold-footed useless swine") and those lies leave Jack's mother with a memory of her son
which makes her happy and proud.

The second external scene
is back at war, likely WW1 as the description speaks of "trenches."  In this scene, the
truth of Jack's death is portrayed.  He was a coward.  He was not a strong nor brave
soldier.  He tried to get sent home and in the end, he died as a result of his fear and
panic.  "Went up" suggests that he attempted to flee the trench, was spotted, and
shot.

The overall tone of this poem is one of bitterness
and irreverence for the subject of war and a fallen soldier.  To choose as the focus for
a war poem someone who was neither respected nor missed ("no one seemed to care except
that lonely woman") is an ironic and somewhat daring thing to do as a poet.  Americans
are taught to revere our soldiers, respect our leaders who are fighting for our freedom,
and support our troops despite what we may think or believe about the concept or
importance of war.

This poem does just the opposite.  It
paints a possibly realistic picture and portrays someone who is both pitiable and
pathetic and yet, not pitied.  The differences in the external scenes heighten the
differences in the internal scenes.  This poem suggests ideas of hypocrisy, irreverence
and disrespect, and possible disagreement with the subject of war, the military, or
protocol.