The repetition of 'how we ended world war one' emphasises the fact that they are probably proud of the actions they've done to end the war. They want to inform you that they did these specific things to prevent the war from going on. It's like a reminder to the reader that the people then did a lot.
The billboard of Doctor T.J Eckleburg is an old advertisement that watches over the valley of ashes. The billboard also represents God watching down and judging society since it watches over a poor city.