The Great Gatsby is significantly impacted in many ways by Fitzgerald's setting of the novel in the time period immediately following World War One.
On a superficial level, Daisy and Gatsby were not able to continue their personal infatuation when Gatsby was sent to Europe as a soldier. The difficulty in even maintaining contact led to Daisy's decision, regretted at the last minute, to marry Tom Buchanan.
On a larger scale, the post-World War One period became known as the Roaring Twenties because of trends and attitudes reflected in many areas of American society. Daisy and Jordan Baker were "flappers" in every sense of the word - they dressed in provocative short dresses with lots of jewelry, loved to dance and drink, enjoyed seemingly meaningless conversations, didn't have careers of any sort.
Gatsby engaged in dealings that were always kept quiet but gave the impression of being related to illegal activities of some sort. The Prohibition Era started in 1919, banning the sale of alcohol, and Tom wondered, "Who is this Gatsby anyhow?...Some big bootlegger?...A lot of these newly rich people are just big bootleggers, you know."