In what ways does Sassoon use irony to express his anger through his poem "The Hero"?

Sassoon's "The Hero" focuses on anger, irony of situation,
and hypocrisy.   

Specifically, concerning anger, the
"Brother Officer" is angry at Jack, a "cold-footed, useless swine."  But on a larger
level, the poem reveals anger at a system that perpetuates
war. 

A boy goes to war.  He is scared to death and does
not represent himself or his country well.  He is blown to bits.  A Colonel writes a
letter to the mother of the soldier, "so nicely."  A fellow soldier goes to his mother
and lies about how brave he was.  The mother is gullible and falls for the nicely
written letter and the lies of the fellow soldier.  The fellow soldier is left
regretting what he's done.

The characters are all in no-win
situations, ironical situations.  None of the characters, with the possible exception of
Jack, can be easily blamed for what they do.  The Colonel and the Fellow Officer
certainly can't tell the mother the truth about her cowardly son.  The mother can't help
but want to believe her son was brave and died nobly.

Yet,
all of them are to be blamed for perpetuating the war machine.  The mother, for
instance, falls for the letter partly because of how nicely it is written.  Those are
her first words after she reads it.  She bows her head because she is choking up and is
embarrassed and hurt, not because she doesn't believe what the Colonel and Fellow
Soldier say.  She believes it and is emotionally
moved. 

The Colonel and the Fellow soldier perpetuate the
myth of a close-knit military that cares for its own, of bravery and valor and heroism. 
When the truth is:

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...how, at last, he
died,

Blown to small bits.  And no one seemed to
care

Except that lonely woman with white
hair.

Irony
abounds:

• the Colonel and Fellow Soldier don't
  tell the truth to the grieving mother


• the mother believes
  the fabricated version of her son's military service, she believes what she wants to
  believe


• this is seemingly how it has to be, at least from
  the military standpoint


• yet this perpetuates the war
  machine, when telling the truth might help the public understand the truth about war and
  therefore make them less likely to support it


• therefore,
  politeness and consideration, if you give the Colonel and the Fellow Officer the benefit
  of the doubt, actually lead to destruction and
  despair.