What is the theme in Cavalry Crossing a Ford by Walt Whitman?

Identity is the main theme of the poem. To the speaker of the poem, the soldiers that are described here have no individual identities but are important in their function as parts of the long, serpentine line of bodies. The speaker appreciates this scene for its artistic harmony, for the way the line of soldiers fits into the overall natural setting. Symbolically, the line of soldiers resembles the river, and the men, "brown-faced," blend into the natural setting like trees. Even the flag that the men follow as a matter of honor and identity is identified with nature by the use of the adjective "snowy." Seen from a distance, these men lose their individual identities, and as a group, the men lose the defining characteristics that separate humans from nature.