In a topic such as American sin and the national transgressions that seem to hover over a brutally beautiful country, I think that Presidnet Lincoln might have encapsulated the feeling the best.  In an 1863 Proclamation that advocated a national day of fasting, the President spoke of "national sins:" "It behooves us then, to humble ourselves before the offended Power, to confess our national sins, and to pray for clemency and forgiveness."  In the Norton Anthology, the mention of "National Sins" is one in which American writers in the 19th Century actively understood the changing moral dynamic in American society.  They spoke to this changing dynamic in their work, often times finding ways to integrate what they saw as moral outrage in their work.  Harriet Beecher Stowe would be one such example as she speaks to the "National Sin" of slavery.  James Fenimore Cooper speaks of another "national sin" in the way Native Americans were treated by White society.  The Mexican- American war with its rationale lying in Manifest Destiny could be seen as another example of a "national sin."  

The idea of a "national sin" is one in which American writers sought to seize a changing moral landscape by injecting their work into the discourse of what constituted morality in the new nation.  Writers like Thoreau, Stowe, and Cooper, to name a few, felt the need to be voices of dissent to a nation that was rapidly industrializing and embracing conformity so easily.  The "national sin" that Lincoln alludes to is one of the most powerful of all in the Civil War, a time in American History where American citizen killed American citizen on a scale that the young nation had never seen.  These examples are what the Norton Anthology embrace when it speaks of "National Sins."  In doing so, one quickly understands the role of literature in helping to guide moral discussion and understanding when ambiguity clouds judgment and full understanding.