What creates the unity of effect in "The Things They Carried"?

You are of course referring to Poe's idea of the "unity of
effect" that a story achieves. His idea was that the author should decide what emotional
effect he or she wants to create in their work and then use every stratagem to achieve
it through, for example, words or phrases that occur and reoccur in a particular work of
literature.

Thinking about the meaning of this phrase when
applied to "The Things They Carried," we can see that O'Brien obviously wants us to feel
immense empathy for the group of soldiers, and in particular for Lieutenant Cross. Again
and again the phrase "They carried" is used to emphasise the way that these soldiers are
burdened in a number of different ways, both physically, with military equipment, but
also mentally and emotionally. Jimmy Cross, for example, carries his photo of Martha
that, in some ways, weighs more than anything else. It is his love for Martha that he
"blames" on Lavender's death as he feels he was distracted. Note what he says after
Lavender's death:

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It was very sad, he thought. The things men
carried inside. The things men did or felt they had to
do.

The text then goes on to
suggest how Jimmy Cross does precisely this by deliberately cutting off his emotional
ties to Martha and focussing dispassionately on the job. We witness the way that war is
dehumanising the soldiers who are supposed to be fighting it through their relationship
with the things they carried, and our hearts ache for these young men, some of them
still even teenagers, who so much is being expected of.