The main theme focuses on French resistance to the German occupiers during the war. During the first half of the story, the narrator explains the background of each of the occupants, with particular emphasis on the petty bourgeois Democrat, Cornudet, who is said to have devised all manner of defences for Rouen. The overriding theme is that while the occupants talk a great deal about resisting the invaders, they are ultimately running away in a cowardly fashion rather than staying in the town. The first section of the story also establishes that the most fiercely patriotic passenger is Boule de Suif herself, an insignificant and unpopular character in Rouen, while the aristocrats and bourgeois are portrayed as happier to betray their country in order to end the war and return to their comfortable lives. In that respect, Maupassant praises the patriotic fervour of the inhabitants of the provinces, in sharp contrast to other French writers of the period who accused provincial French citizens of being apathetic and cowardly.
Boule de Suif's personal resistance grows throughout the story; when the coach is stopped by the Germans at the village of Tôtes, the other passengers meekly follow the officer's orders while Boule de Suif refuses to co-operate as easily. Boule de Suif's resistance to the officer's sexual advances again shows her patriotism, something which is noticed by the other characters, who comment that although it is Boule de Suif's job to sleep with men, she patriotically refuses to allow herself to be conquered by the German officer.
Like Maupassant's other short stories on the Franco-Prussian War, he tends to stereotype the German soldiers. The troops holding Rouen are hinted at as dull and slow-witted. The German officer at the inn is portrayed in the same way as Maupassant depicts German officers throughout his stories; the officer is shown as being arrogant, morally dubious, and unfeeling. The description of the officer in his room at the inn suggests that he is pointlessly destructive, uncultured, and boorish. At the same time, there are passages that describe how German troops get about their daily life and long to return home to their own families.
The theme of class barrier is also tackled in the story. Throughout the story, Maupassant portrays the inhabitants of the coach in various disparaging ways. The aristocratic Comte and Comtesse are revealed to be weak and cowardly in spite of their position as the chief dignitaries of Rouen. The manufacturer and his wife are constantly portrayed as greedy and materialistic, and the manufacturer's wife in particular is always shown to be shocked whenever her husband spends any money.
The petit bourgeois wine-seller and his wife are shown as corrupt and morally reprehensible, the most likely of the party to betray their country simply to return to a life of greed in peace. The two nuns travelling in the coach are at first portrayed as quiet and subservient to God, and later show themselves as fiery, patriotic, and doing more for their country than the other occupants of the coach: the nuns claim to be travelling to a military hospital to treat wounded French soldiers, thus offering the deciding argument towards persuading Boule de Suif to abandon her resistance. The narrator offers to excuse their crafty argumentation as accidental stupidity, but the nuns' base behavior as they fail to share food with the courtesan raises a question mark if not necessarily on their story then on their altruistic motivation. Cornudet is repeatedly shown as a man who is little more than a drunken, lecherous, and cowardly man who is not prepared to stand up for his vicious anti-German beliefs when the time comes. In contrast to all of these is Boule de Suif herself, revealed to be the most fiercely patriotic, kind-hearted, and morally admirable character, which Maupassant contrasts with the hypocrisy and snobbery of the other travellers.
Despite being shunned by the other occupants at first, she gladly shares her picnic basket with the hungry occupants of the coach, but at the end of the novel, when she has no food for the other half of the journey, the coach's other occupants refuse to share their food with her, an ingratitude made even worse by the fact that it was Boule de Suif's personal sacrifice that allowed them to leave. Her self-sacrifice in sleeping with the German officer underlines her personal courage and the blind hypocrisy of the other travellers; the travellers go to great lengths to persuade Boule de Suif to sleep with the officer in order that he will let the coach continue its journey, and the travellers fill Boule de Suif's head with arguments, arguing that it is for the good of the country; that it is not morally wrong to sleep with the officer to let the travellers leave; and that the longer she waits, the more young French soldiers will die as the nuns are not there to look after them.
When Boule de Suif gives in and sleeps with the officer, the rest of the travellers throw a party without her, and when the coach finally leaves the next morning, they treat her with utter disgust and contempt although she has freed them and they had induced her to lose her dignity.
