Joseph Loconte is an associate professor of history at the King¹s College and the author of "A Hobbit, a Wardrobe, and a Great War: How J.R.R. Tolkien and C.S. Lewis Rediscovered Faith, Friendship, and Heroism in the Cataclysm of 1914-1918."

Oxford, England (CNN) In his controversial encyclical on climate change, Pope Francis delivered a scathing critique of environmental degradation and called for "an ecological conversion" among fellow Christians. A century earlier, however, another environmental debate prompted its own version of soul-searching among the faithful.

By the summer of 1915, a year into the cataclysm of the First World War, the combatant nations had settled into a deadlock of industrialized slaughter. Millions of soldiers already had perished in the trenches and barbed wire and mortar fire along the Western Front, with nothing to show for it.

"Injuries were wrought to the structure of human society which a century will not efface" wrote Winston Churchill, "and which may conceivably prove fatal to the present civilization."

Often neglected in the story, though, is how the Great War destroyed not only countless soldiers and civilians, but also their environment. Across much of Europe, entire forests disappeared; fields and farms were turned into craters.

Describing a scene near Amiens, author Vera Brittain, looking for the grave of her fiancé, found "grotesque trunks of skeleton trees, with their stripped, shattered branches still pointing to heaven in grim protest against man's ruthless cruelty to nature as well as man."

World War I produced, in essence, an environmental holocaust.

This aspect of the conflict left a deep impression on two Christian authors and Oxford dons, J.R.R. Tolkien and C.S. Lewis. Having personally experienced the awful prodigy of modern industry and technology -- both men fought in the trenches in France -- they enlisted nature itself as a protagonist in their epic stories of good vs. evil.

Even before the war, Tolkien and Lewis had come to resent the encroachment of industrial life into rural England. Tolkien lamented "the tragedy and despair of all machinery laid bare," meaning the modern attempt to enhance our control over the world around us, regardless of the consequences.

In "The Lord of the Rings," Saruman the wizard "has a mind for metal and wheels; and he does not care for growing things, except as far as they serve him for the moment." The hateful realm of Mordor is sustained by its black engines and factories.

Likewise, Lewis viewed respect for nature as intrinsic to human happiness.

In "The Chronicles of Narnia," his series of books for children, the various animals play a central role in the story. The smallest of creatures -- such as a mouse named Reepicheep -- display the greatest of human virtues.

As biographer Alister McGrath observes: "Lewis' portrayal of animal characters in Narnia is partly a protest against shallow assertions of humanity's right to do what it pleases with nature."

The experience of war deepened this sensibility. Both men enlisted as officers in the British Expeditionary Force and saw intense fighting at the front.

Lewis was injured by mortar fire -- the shell killed his sergeant standing a few yards away -- and was shipped back to England to recover. "My memories of the last war," he wrote, "haunted my dreams for years."

Tolkien survived the ferocious Battle of the Somme, but contracted trench fever and was taken out of harm's way. In the horror of the Somme he was given a vision of Mordor: the "dead grasses and rotting weeds" and "a land defiled, diseased beyond healing."

As Tolkien acknowledged years later, the Dead Marshes "owe something to Northern France after the Battle of the Somme."

The two great authors met at Oxford in 1926, where they discovered a mutual love for mythology and English literature. Tolkien, a devoted Catholic, helped convert Lewis to Christianity. Lewis persuaded Tolkien to pursue his story about hobbits and Middle-earth.

Their influence on each other's literary imagination was subtle, yet profound. In the climactic scenes in both of their epic works -- stories framed by a great war -- nature itself is caught up in the conflict.

In Lewis' "Prince Caspian," the character Trufflehunter explains to Caspian that it will be difficult to wake the spirits of the trees in the battle against Miraz, the unlawful King of Narnia.

"We have no power over them. Since the Humans came into the land, felling forests and defiling streams, the Dryads and Naiads have sunk into deep sleep." Yet the war cannot be won without their help, and Aslan, the great Lion, summons them to join the battle: the "woods on the move."

Tolkien's humanoid trees, the Ents, are among the most memorable figures in fantasy. Led by Treebeard, the oldest living creature in Middle-earth, the Ents were created as guardians of the forest.

Earlier wars had decimated the land, forcing the Ents to confine themselves to Fangorn Forest, where they hoped to avoid the War of the Ring. But Sauron's advance compels them to abandon their moral neutrality.

"A thing is about to happen which has not happened since the Elder Days," explains Gandalf. "The Ents are going to wake up and find that they are strong."

At the start of the 20th century, when Tolkien and Lewis began their Oxford careers, enthusiasm over "the conquest of nature" was at a fever pitch in the industrialized West. The fever raged with horrific fury during the Great War.

In the Christian imagination of these authors, the assault on nature carried spiritual significance: Man's sins against nature will not go unpunished, and nature will take its revenge.