The title means "in defence of my poetry" and is often viewed as a rebuttal to a remark in Robert Graves' letter "for God's sake cheer up and write more optimistically - the war's not ended yet but a poet should have a spirit above wars.
The poem describes some of the horrors of war and how this leads to a lack of emotion and a desensitisation to death.
However the key message of the poem is revealed in the final two stanzas criticizing "you" at home (contemporary readers) for using war propaganda and images as a form of entertainment "These men are worth/ Your tears: You are not worth their merriment."
The poem begins and concludes as follows: I, too, saw God through mud— The mud that cracked on cheeks when wretches smiled.
War brought more glory to their eyes than blood, And gave their laughs more glee than shakes a child.