The Spanish Civil War, a military rebellion supported by Hitler and Mussolini, attracted the greatest writers of the age. Among them were Ernest Hemingway, George Orwell, Andre Malraux, Arthur Koestler, Langston Hughes, and Martha Gellhorn. They returned to their homelands to warn the world about a war of fascist aggression looming on the horizon.

Spain's cause drew 35,000 volunteers from 52 countries, including 2,800 Americans who formed the Abraham Lincoln Brigade. Eight hundred...