This excellent selection of war-time stories is interesting in the way that the major symbols that are used are actually people rather than objects. Consider, for example the Vietnamese man that O'Brien thinks he killed in "The Man I Killed." It is clear that this soldier serves as a symbol of humanity's guilt concerning the atrocities of war. Consider how in this story this man is given an imagined past and how the tale is written in the third person (unlike nearly all the other tales in this collection) so as to distance the author from the crime. In particular, the narrative focuses on the star-shaped hole that is in the place of the soldier's eye:
He was a slim, dead, almost dainty young man of about twenty. He lay with one leg bent beneath him, his haw in his throat, his face neither expressive nor inexpressive. One eye was shut. The other was a star-shaped hole.
The way that this description is refered to again and again throughout this tale and the fantasies that the author constructs about this anonymous soldier's life, which are markedly similar to the author's own life in some respects, reveals the trememndous guilt that the author feels, even though the narrative never directly addresses his own feelings of remorse.
You might like to consider how other characters are important symbols, such as Kathleen and Linda. I hope this helps and good luck!