I think that one of the most memorable phrases in Lee's letter to his son is the conveyance of absolute sadness about the predicament of the nation on the eve of Civil War. Lee's statement of personal mourning is undeniable:
Still, a Union that can only be maintained by swords and bayonets, and in which strife and civil war are to take the place of brotherly love and kindness, has no charm for me. I shall mourn for my country and for the welfare and progress of mankind.
Such a line is memorable because it challenges the preconceived notion that every prominent Southerner embraced secession as the answer. Lee was quite prophetic in being able to foresee the consequences of the Civil War, a war in which there was no real winner given the massive amount of bloodshed and upheaval experienced.
Another memorable line from Lee's letter would be the concluding line in which Lee articulates what he sees as the essence of duty and responsibility. The closing line of the letter to his son displays the extent to which Lee sees duty as a part of his being: "I shall return to my native state and share the miseries of my people; and, save in defense, will draw my sword on none." For Lee, there is an honor to be upheld. Lee defines honor as doing what one knows is an insurmountably difficult task. As Lee has already defined secession in the most destructive of terms, he ends his letter with the understanding that he will embrace what his duty compels him to accept, regardless of the amount of pain it will carry. Such a notion is memorable because it truly adds a complex dimension to the leader of the Southern army.
In terms of "The Gettysburg Address," there are many memorable lines. Working backwards, the last line of the speech is one of the most memorable in American History:
...we here highly resolve that these dead shall not have died in vain, that this nation, under God, shall have a new birth of freedom and that government of the people, by the people, for the people, shall not perish from the earth.
Lincoln's conception of American democracy as a government comprised of the people's voice is the very essence of republicanism, popular sovereignty, and nearly every principle of the United States Constitution. Lincoln's ability to transform public thinking about the war into something so profound is memorable and a testament to why the speech is so essential in the study of American History.
Another memorable line features the promises and possibilities that define "America." The opening line of the speech is memorable in this regard: "Four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal." Lincoln's line here is memorable because it defines American identity. Lincoln understands that the essence of equality is what underscores the foundation of America. It is the basis of American institutions such as government, law, and social configuration. In its invocation, Lincoln is able to connect the Civil War to such a notion of American identity, making it extremely memorable.