A main theme is the travesty of death that occurs in war. In the peom Whitman laments over the fact that "my enemy is dead, a man divine as myself is dead"; he steps over the boundaries of war and enemies to mourn the death of a man. Forget the fact that he was an enemy; he was a man, just like Whitman himself. And essentially, that is what war is-killing other people that are much like ourselves. It is a sad truth of war. Whitman mourns it, and standing over his coffin, he states, "I...draw near, bend down and touch lightly with my lips the white face in the coffin," giving a big of respect and homage to the man who he killed in war. He respects him by mourning his death in the first place, then placing a kiss on the face, as a kind-of apology.
War is something that happens often in our world, and Whitman recognizes this, that "carnage must in time be utterly lost" as "Death and Night incessantly wash again, and ever again, this soil'd world." He is saying that carnage happens, but death and continuing time washes it away. But he is still left there to mourn the enemy he killed.