Answer:
The war.
Explanation:
F. Scott Fitzgerald's "The Great Gatsby" revolves around the character of Jay Gatsby and his 'lost American dream'. Though narrated by another character, Nick Carraway, the novel focuses on Gatsby, his life, his desire for Daisy, the theme of status, greed, betrayal, lost love, etc.
In Chapter 3, Nick had gone to Gatsby's party after being invited. Though he knows his neighbor's name is Gatsby and that he would often throw parties, he hadn't actually met the man himself. While there at the party, he was conversing with Jordan at a table where there was a <em>"man of about [his] age"</em>. That man started a conversation with him, asking if he had been <em>"in the Third Division during the war"</em>, to which Nick replied that he <em>"was in the Ninth Machine-Gun Battalion."</em> The man then declared that he <em>"was in the Seventh Infantry until June nineteen-eighteen." </em>Shortly after this encounter, Nick discovered that the man was Gatsby himself after Gatsby remarked, <em>"I'm Gatsby."</em>
