For Whom the Bell Tolls is narrated by a third-person omniscient narrator. Typically, the story is told from Jordan's perspective, but the point of view shifts to El Sordo's during the ambush, as well as Karkov's during Jordan's flashbacks to Madrid. As such, tone in the novel relies heavily on whichever character's stream-of-consciousness narration Hemingway highlights (usually Jordan's). Generally, this tone is one of uncertainty and instability—war, after all, is destabilizing, and the novel's central characters often appear torn as to how they should think and feel.
Despite the uncertainty of war and Jordan's lack of understanding as to how he should feel, For Whom the Bell Tolls concludes with a tone of optimism and gratitude. Note the following passage from Chapter 43:
And if you wait and hold them up even a little while or just get the officer that may make all the difference. One thing well done can make—
All right, he said. And he lay very quietly and tried to hold on to himself that he felt slipping away from himself [...].
A dying Jordan concludes his thought spiral with a tone of optimism and self-efficacy, confident that his dying efforts may make the difference for his friends, comrades, and lover.