There are many ways to identify the tone of a text. A writer often expresses tone through syntax, diction, point of view, and perspective to name a few. I'll focus on how the perspective and point of view inform the tone of the story, and conclude by looking at how diction, specifically the word "dark," redirects the mood of the story.

“Guests of the Nation” tells a story of three Irish rebels who are charged with holding two Englishmen prisoner. The Irish and English characters form a bond over cards and companionship, a bond soon fragmented when the unspoken leader of the Irish rebels, Jeremiah Donovan, reveals that they will likely execute the prisoners.

The overarching tone of “Guests of the Nation” is informal. The story is told in 1st person, giving the reader a window into the narrator's thoughts and feelings. A 2nd person narration would lend a sense of informality, but might fall short in evoking such a strong sense of nostalgia. A 3rd person narration might evoke nostalgia but might make the reader feel less invested in the drama, and perhaps less nostalgic.

As the narrative progresses, it feels significant that O'Connor chose to tell the story through Bonaparte's perspective rather than through the other characters', as Bonaparte's personality is more amiable and level-headed. The reader perceives a joyful nostalgia in the way Bonaparte describes playing cards with Belcher, Noble, and 'Awkins:

Sometimes Jeremiah Donovan would come up of an evening and supervise the play, and grow excited over 'Awkins's cards (which he always played badly), and shout at him as if he was one of our own, 'Ach, you divil you, why didn't you play the tray?' But, ordinarily, Jermiah was a sober and contented poor devil like the big Englishman Belcher. (371)

Bonaparte excuses Jeremiah's temper in this passage as an exception to his true nature. If this story were told through Jeremiah's perspective, it's likely that the tone would be more formal and laced with the coldness and sense of duty that's often characteristic of military leadership. Later we learn that Jeremiah might harbor some disdain for the prisoners, but for Bonaparte, “it was a treat to see how Belcher got off with the woman of the house [they] were staying in,” (372). Bonaparte makes little effort to conceal his fondness for the prisoners, and it seems Noble shares this sentiment.

The tone begins to shift in part II when we learn that the Irish rebels might have to execute the English prisoners. The news is foreshadowed when “a terrible dispute [blows] up late in the evening between 'Awkins and Noble, about capitalists and priests and love for [one's] own country,” (373). This argument isn't excused with acceptance and nostalgia as in the first section. In the opening argument, Jeremiah “shouts at ['Awkins] as if he was one of [their] own,” but this dispute is “terrible,” and the argument continues for a couple of pages. By the end of the story, the tone has shifted to helplessness, fear, guilt, and disillusionment.

To help facilitate this shift, O'Connor emphasizes darkness in the last two sections: A “dark presentiment” crosses Bonaparte's mind just before Jeremiah confirms that they're to execute the prisoners; the house is “pitch dark” when the woman of the house tries to convince Jeremiah to let them stay; “no one thought of lighting the lamp, and in the darkness the two Englishmen fetched their khaki topcoats and said goodbye”; a “great sadness over[takes Bonaparte's mind] as they walk the prisoners “in the darkness”; Feeney stands “somewhere in the darkness behind” and “somehow the picture of the two of them so silent in the boglands was like the pain of death in [Bonaparte's] heart; In Belcher's last moments, he reflects on the argument he'd had with 'Awkins over religion, and how “last night 'e was all in the dark,” (377-380). There's no mention of the work “dark” until the tonal shift. Often a symbol for death, remorse, and suffering, O'Connor employs darkness to shift the tone from a fond sense of nostalgia to helplessness, fear, and guilt.

O'Connor, Frank. “Guests of the Nation.” The Oxford Book of Short Stories, edited by V. S. Pritchett, Oxford University Press Inc., 1981, pp. 371-381.

There are four tones, each claiming one of the four sections of O'Connor's short story:  contentment, disillusion, tension, and regret.  

Section 1 finds Bonaparte, the Irish Rebel and narrator in a jovial mood with his "prisoners," two Englishmen named Belcher and Hawkins.  So genial are the three, (as well as the other Irish Rebel, Noble), that Bonaparte wonders  "what the hell we wanted guarding them at all for" and "What use are these fellows to us?"

He soon discovers that the Englishmen are not prisoners, but hostages.  Donovan, the lead officer says, "The enemy have prisoners belonging to us and now they're talking of shooting them.  If they shoot our prisoners, we'll shoot theirs...what else did you think we were keeping them for?"  (Section II).

Despite his objections, Bonaparte leads his friends away.  Now aware of their fate, the Englishmen beg, "What had he done to use?  Weren't we all chums?"  (Section III). 

Donovan shoots Hawkins but he does not immediately die.  Belcher ties a handkerchief around his own eyes; too small, he accepts another from Bonaparte, who shoots him.

Section IV closes with Bonaparte's nauseating sense of betrayal and guilt.  "I was very small and very lost and lonely like a child astray in the snow.  And  anything that happened to me afterwards, I never felt the same about again."