The themes of Carl Sandburg's poem, "Grass," include humans kill each other in war and let nature cover it up and that nature continues even during wartime. Other themes include people not remembering the lessons that history teaches and people forget the heroes that fall during war.
When people do not remember what history teaches them, they repeat mistakes and may find themselves at war again. The grass eventually covers the battlefields and makes it appear as if a war never took place in that location. The narrator of the poem is grass and it is sharing a point of view in the first person.