This short section tells the story of Amory's time at war solely through two letters and a short narration. The first letter, dated January 1918, is from Monsignor Darcy to second lieutenant Amory, stationed in Long Island. Darcy rages at the violence of the war and draws parallels through history. He observes that he has become an old man and implies that he wishes he were Amory's father. He praises Amory's dutiful and sober entrance into the war as quite admirable and noble. Feeling a great bond between them, Darcy praises their similar faith and simplicity. Drawing most on their common Celtic ancestry, Darcy includes a poem that contains a number of references to Gaelic mythology. He closes with his fear that one of the two of them may not survive the war and says that he feels that Amory is, in a way, a reincarnation of himself.
Amory, on the deck of his boat heading for Europe, writes a short poem on his departure. It is a descriptive piece that hints at the regret the speaker will feel about the "futile years" of the war.
The last piece of this section is a letter sent from Brest, Germany in January 1919, in which Amory tells Tom, who is still in America, his plans for after the war. Amory hopes to get an apartment with Tom and Alec Connage in Manhattan and is considering entering politics, complaining that while the best of the British enter politics, the best of the Americans try only to make money. With regard to money, Amory mentions in passing that Beatrice has passed away and, he grumbles, left half of her money to the church. Amory writes that he "would write immortal literature if he were sure enough about anything to risk telling anyone else about it." He laments the deaths of Kerry and Jesse in the war and wonders where Burne is. He remarks how more men have discovered Paris than "G-d" (God) in Europe, and says that he fears growing fat or falling in love. Upon his return, he wishes to lead a "contemplative, emotionless life."
This short section is significant, starting even with its title, "Interlude." World War I, called the first modern war because of the new weapons and tactics, claimed an unprecedented number of lives, devastated Europe, and significantly altered the cultural world that produced it. Yet, Fitzgerald refers to the war simply as an interlude, wholly stripping it of any real weight and giving it the air of a vacation or hiatus. This decision reflects the blasé way in which Amory himself views the war. He seems to think it affected him very little, but he realizes later that it had a greater impact on him than he initially suspected. The disregard for the importance of the war accents Amory's still strong egotism and also the prevailing apathy that many people felt following the war, as though nothing mattered anymore.
Darcy's letter displays the strength of his bond with Amory--wishing he was Amory's father and suggesting that Amory is his reincarnation. Darcy is a priest and believes that Amory shares his faith in God, but simply has not discovered it yet. The more likely truth is that Amory has not found anything in which to have faith. Yet, Darcy applauds the way in which Amory entered the war without emotion, out of a sense of duty. While this undermines Amory's search for a passionate outlet, it hints that he has achieved some of the qualities of the ideal gentleman that he saw in Dick Humbird.
Darcy cites their shared Celtic ancestry as a main link between the two in this letter, stressing it with the poem he includes. This mention of European ancestry may in some way provide a reason for Amory's involvement in the war, but serves mostly to show Amory's conception of himself as having Celtic roots.