The comment about General Lee helps to situate the book in time—it’s taking place after the American Civil War, which ended in 1865. The banter that follows suggests the era’s quickly evolving and often contentious ideas about race and identity. It’s not necessarily clear who is or is not an American. Call’s abrupt departure suggests his disinterest in the question, hinting that to him, a person’s character is more important than their country of origin or skin color.