For Whom the Bell Tolls is a classic example of a war novel, situated solidly within the late modernist genre. Late modernism bears many similarities to its earlier counterparts, representing the tail end of an era of tumultuous change. The turn of the 20th century saw significant technological, scientific, and cultural change; the early modernist period constituted a response to these changes, an adjustment to a world operating by a new set of rules. Late modernism, on the other hand, represents these same changes made ubiquitous. By the time WWII begins, the existential horror of weapons of mass destruction has become a grim reality, rather than a horrific, emergent one.
In addition to being a late modernist novel, For Whom the Bell Tolls is a war novel. War novels often contain themes of loss, trauma, and inevitability: soldiers feel resigned to their fates, and may end up having little control over their own deaths. Modernist and late modernist war novels, in particular, fixate on the futility of war and its cyclical nature. Many soldiers post-WWI became incredibly disillusioned with the image of war as "glorious," having witnessed the advent of mass-destructive weapons. For Whom the Bell Tolls represents the grim disillusionment with the violence of late modernity.