“The real war,” Walt Whitman famously wrote about the Civil War, “won’t get into the books.” He was thinking about the indescribable horror of that conflict, of a trauma that could not be expressed in language. Of course, Americans took up his challenge. We have great works like The Red Badge of Courage to show for it. Other wars have similarly inspired cultural productions. The modern age of film has given us classics like They Were Expendable and Apocalypse Now, among many others. But what about non-“war” genres? Were they affected? Yes, says W. Scott Poole, in his recent book Wasteland: The Great War and the Origins of Modern Horror. World War I, apparently, revolutionized the horror genre.

Explains Poole, the unprecedented carnage of WWI (which ended the lives of some 40 million people worldwide) bled directly into popular culture. The trauma of seeing bodies disassembled by weapons, the mystery of post-traumatic stress disorder (PTSD; called “shell shock” at the time), the obliteration of landscapes, and the fear of annihilation from gas and explosives and machine guns all wormed their way into the psyche of humankind. Afterward, horror films looked different. They became surreal, obsessed with dark shadows and the supernatural, and filled with new monsters.

Take Frankenstein, directed by James Whale, who, as a British officer, had been captured on the Western Front and sent to a German prison camp for more than a year. He did not speak of his experiences, but his films, arguably, reflected them. The monster in Whale’s 1931 Frankenstein is not like the literate, expressive creature of Mary Shelley’s source novel. It is, rather, a lumbering, misshapen, mute beast. He walks as if shell-shocked, in a trance, and destroys everything in sight, including an innocent little girl. Says Poole, this Frankenstein’s monster reflects the experience of WWI trauma and the catastrophic damage—physical and emotional—exacted by the conflict. Film, he explains, “allowed (and still allows) audiences to talk about trauma in an oblique way.” Whale, perhaps, offered an oblique expression of the war, a parable of his pain.

Or take Phantom of the Opera. This silent 1925 classic, based on a French novel, tells the story of a mysterious figure who haunts an opera company. The film’s most famous scene is the moment when the beautiful chanteuse Christine unmasks the Phantom (played by Lon Chaney). Audiences suddenly saw a hideous visage, a face that looks partially stripped off. This effect, says Poole, must have resonated with an audience grown familiar with seeing ravaged war survivors, many of whom had ghastly facial damage.

Other wars have also lurched into pop culture in oblique ways. The Second World War, for instance, made the noir style popular. With dark, obscuring shadows, duplicitous characters, and occasional bleak endings, these gritty films seemed to revel in the atmosphere of disillusionment and hard-to-process war memories. They also added an undeniable new horror aesthetic to standard police procedurals and mystery flicks. The soon-to-follow nuclear fears of the Cold War gave us giant irradiated ants and monstrous reptiles.

The Vietnam War perhaps offers the most recent major shift in horror. Regarding the documentary The American Nightmare, director Adam Simon explains how rapidly celluloid became gorier in the Sixties. ''What happened in part,'' says Simon, ''was simply that there were images being delivered to American living rooms that would not have been allowed on the screens of their movie theaters without an X rating. And the self-evident contradiction of that, in part, was what broke through the barriers.” Scenes of wartime carnage made it onto the nightly news. Filmmakers felt a need to respond in kind.

Night of the Living Dead, which premiered just after the catastrophic Tet Offensive in 1968, features a group of youngsters getting overrun by bloodthirsty zombies. Here, audiences saw, for the first time, humanoid creatures literally consuming human guts. Tom Savini, who did the makeup effects for Romero’s 1978 zombie follow-up Dawn of the Dead, was a U.S. Army photographer in Vietnam. He’d seen plenty of real gore, and his effect work reflected the horrors that he witnessed through the camera lens.

War always makes it “into the books” (and films). It cannot help but infiltrate the popular mind. We often think of novels, memoirs, and films that directly treat war experiences as the basic reaction. But perhaps the signal feature of war is horror. It is in this genre that we should look for the creeping skeletal hand of combat death.