Swallows (Symbol)
The swallows are the central symbol in the poem because they are used to represent different things. The introduction of the birds as symbols is expressed by the line “The swallows fly low” which brings to mind the image of a flag flying at half-mast to honor the dead. This image is part of poetic traditions dating back to 17th century England. The silence and low altitude of the birds provide imagery consistent with this use of the swallows as symbolically representing the flags of both armies. The swallows also serve as a peaceful counterpoint to the bullets and cannonballs that were flying over the soldiers shortly before the poem takes place.
April Rain (Symbol)
As a matter of historical fact, the Battle of Shiloh did actually occur on a rainy weekend, but here the rain providing solace to the parched victims on the field takes on a deeper symbolic significance. In traditional poetic imagery, the rain falling in April heralds the arrival of spring and rebirth. The rain acts to symbolically cleanse the blood-soaked battlefield and by extension the horror and trauma of the war as a whole. As befits the mood of the poem overall, this soothing moment is marked by its fleetingness and limitations. The rain can only provide a small relief.
Religion (Motif)
As the poem progresses it grapples with spirituality's role in the dying soldiers' final journey. A log church sits near the battlefield and, as the speaker describes it, the church comes to harbor the spoken words (both prayers and groans) of the dying men around it. What the speaker indicates here is both a passing hope of salvation for these men as well as a kind of unity occurring, as they are stripped of their political allegiances in the face of death. There is also an element of sacrilege here; the church is in incredibly close proximity to men inflicting horrors upon one another. Even if the church provides a moment of respite, religious overtones are also a cold reminder of how far circumstances pushed these men away from the guidance of faith.