...love is no longer possible, for a battery of reasons, both practical and allegorical. On the surface, since the Monterists hold the town Antonia might be physically dead. Even if alive, he could not return to her, both because of the danger to himself and, one senses, because of the humiliation attending the (perceived) failure of his political gambit.

Allegorically, Antonia represents idealism, and on this more crucial level it is idealism that Decoud cannot face, even if it has survived. Not even in the form of love, not even to save the self, can he return to the illusion. This is his final surrender to skepticism -- and fittingly, a rejection of Antonia stands at the end of it. He is now completely lost in all-embracing futility (note that "exertion" can apply to mental effort as well as physical).