In “The Defender of the Faith,” what does Sergeant Nathan Marx mean when he says that he had “grown an infantryman’s heart, which, like his feet, at first aches and swells, but finally grows horny enough for him to travel the weirdest paths without feeling a thing”? What kind of figure of speech does Philip Roth use here? How does that figure of speech relate to the themes of the short story?

Nathan Marx has served in the army during World War II for over three years, and he has seen horrible things. He has raced with the rest of the Ninth Army across Germany. Later in the story, he recalls some of what he saw in the forests of Belgium and in Germany: people dying, soldiers burning books from German farmhouses to survive, and the destroyed cities of Europe. To survive, he has hardened his heart. In this quotation, which is a simile, he compares his hardened heart to his feet. At first, a soldier's feet are raw and become sore from marching; however, over time, they harden and become callused so that they feel no pain. Like his feet, his heart is immune to pain because he has become so tough.
This figure of speech relates to the rest of the story because Marx comes to disdain and then hate Sheldon Grossbart, who tries to carve a softer path for himself in the army. Grossbart thinks Marx will protect him because they are both Jewish, but Marx rejects Grossbart's pleas for sympathy when he realizes that Grossbart has lied to him. Marx's first priority is thinking about the goals of the army and winning the war. He thinks like a hardened soldier, not like a civilian who has the luxury of feeling pity. In the army, Marx is no longer capable of sympathy but can only feel vindictive.