How much does the reader need to understand about the allusions in "Grass" to appreciate their importance to the literal meaning of the poem?
Obviously, a reader who does not know any of the place names mentioned by Sandburg would probably not know what this poem is about. All are places where battles occurred in European and American wars—battles in which the carnage, the number of dead and wounded, was especially great. Yet one could argue that from simply the mention of "piling high" the bodies, most readers would be able to guess the meaning of "Grass."
However, the poem would still not fully resonate with someone who does not recognize these references as cultural tropes, symbols of our history. Of the places mentioned, Gettysburg and Waterloo are the most famous. Sandburg is using our recognition of historical symbols to get across the meaning of the poem in a visceral, emotional way. The lines also pose the question of whether warfare actually accomplishes anything. "The grass," in doing its work, is engaged in an endless process of destruction and renewal. The speaker, the grass itself, seems to know and accept this fundamental truth.
The point about warfare is significant because, while Waterloo and Gettysburg were victories for the "side" in battle with which most people today would identify, Austerlitz was a victory for Napoleon (in other words, it had the opposite outcome of Waterloo). The grass, the eternal force which witnesses all of this, is thus indifferent to which side is "right" or "wrong" in battle, and the implication may be that humanity's differences, which are settled by killing, are inconsequential in the scheme of things—or that man's use of warfare to resolve these issues is foolish and counterproductive.
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