During which military conflict does Ernest Hemingway's novel "For Whom the Bell Tolls" take place?

For Whom the Bell Tolls is a novel by Ernest Hemingway published in 1940. It tells the story of Robert Jordan, a young American in the International Brigades attached to a republican guerrilla unit during the Spanish Civil War. The novel graphically describes the brutality of the civil war in Spain during this time. It draws on Hemingway's own experiences in the Spanish Civil War as a reporter for the North American Newspaper Alliance.