A Farewell to Arms by Ernest Hemingway
From Wikipedia: "A Farewell to Arms is a novel written by Ernest Hemingway set during the Italian campaign of World War I. The book, published in 1929, is a first-person account of American Frederic Henry, serving as a Lieutenant ("Tenente") in the ambulance corps of the Italian Army. The title is taken from a poem by 16th-century English dramatist George Peele.
A Farewell to Arms focuses on a romance between Henry and a British nurse, Catherine Barkley, against the backdrop of World War I, cynical soldiers, fighting and the displacement of populations. The publication of this, Hemingway's bleakest novel, cemented his stature as a modern American writer, became his first best-seller, and is described by biographer Michael Reynolds as "the premier American war novel from that debacle [World War I]"