Just like the tactics of the great generals, by laying out the square in advance with clear, orderly lines, you insulate yourself from the chaos of improvisation. You mark the boundaries now so you don’t have to later, and excellence is achieved simply by filling them in with your sentences. Each paragraph is given a singular purpose and its only duty is fulfillment. Like I said earlier, with this structure you place the reader in the middle of the square, protected by all sides, and methodically move them forward, defending doubts and objections as they arise. And that is a great essay.
Fitzgerald connects symbolism and motif to the idea of the American Dream by using them to demonstrate what he saw as the perversion and loss of the true American Dream. Originally the American Dream was centered on the ideological: discovery of self and opportunity; individualism instead of recognition by rigid class definition; the "pursuit of happiness" as proclaimed in the Declaration of Independence. Fitzgerald uses symbols and motifs to tie decadence, decay, immorality and greed to the aspirations of Americans like Daisy, Tom, Baker and Gatsby, whom Daisy's greed has corrupted to the point of pursuing criminal activity that he might attain the things that will attract her and lure her to him.
A thesis for this might be something like: "Fitzgerald uses symbols and motifs such as The Valley of Ashes and weather to show that America had materialized its original Dream of high ideals such as individualism and happiness."