Task: Question Generation
Input (English):  In this novel the focus shifts from John Carter, Warlord of Mars, and Dejah Thoris of Helium, protagonists of the first three books in the series, to their son, Carthoris, prince of Helium, and Thuvia, princess of Ptarth. Helium and Ptarth are both prominent Barsoomian city state/empires, and both Carthoris and Thuvia were secondary characters in the previous novel.
Its plot devices are similar to the previous Martian novels, involving the kidnapping of a Martian princess. This time John Carter's son Carthoris is implicated. It does however have some inventive and original ideas, including an autopilot and collision detection device for Martian fliers, and the creation of the Lotharians, a race of ancient martians who have become adept at telepathic projection, able to create imaginary warriors that can kill, and sustain themselves through thought alone.Carthoris is madly in love with Thuvia. This love was foreshadowed at the end of the previous novel. Unfortunately Thuvia is promised to Kulan Tith, Jeddak of Kaol. On Barsoom nothing can break an engagement between a man and woman except death, although the new suitor may not cause that death. Thus it is that Thuvia will have none of him. This situation leaves Carthoris in a predicament.
As Thuvia suffers the common Burroughsian heroine's fate of being kidnapped and in need of rescue, Carthoris' goal is abetted by circumstances. Thus he sets out to find the love of his life. His craft is sabotaged and he finds himself deep in the undiscovered south of Barsoom, in the ruins of ancient Aanthor. Thuvia's kidnappers, the Dusar, have taken her there as well, and Carthoris is just in time to spot Thuvia and her kidnappers under assault by a green man of the hordes of Torquas. Carthoris leaps to her rescue in the style of his father.
The rescue takes Carthoris and his love to ancient Lothar, home of an ancient fair-skinned human race gifted with the ability to create lifelike phantasms from pure thought. They habitually use large numbers of phantom bowmen paired with real and phantom banths (Barsoomian lions) to defend themselves from the hordes of Torquas.
The kidnapping of Thuvia is done in such a way that Carthoris is blamed. This ignites a war between the red nations of Barsoom. Carthoris must try to be back in time with Thuvia to stop the war from breaking loose. Carthoris wonders if his love will ever be requited by the promised Thuvia.
Output (English): What is the only thing that can break an engagement between a man and woman on Barsoom?