Arthur Koestler was arrested by Francisco Franco’s Nationalist forces in the city of Málaga on February 9, 1937. Koestler had come to Spain, in the midst of the Civil War, as a correspondent for a British paper called the News Chronicle, and although Málaga had been abandoned by Republican troops and most of its inhabitants several days earlier, and although the reporters Koestler was travelling with had fled, he had stayed behind. Why is not clear. Michael Scammell, in his compendious new biography, “Koestler: The Literary and Political Odyssey of a Twentieth-Century Skeptic” (Random House; $35), suggests a number of possibilities: Koestler felt loyal to the acting British consul, Sir Peter Chalmers Mitchell, who had a house in the city and with whom he had become friendly; he was disgusted by the cowardice of the deserters and wanted to show bravery himself; he couldn’t face the thought of leaving his typewriter behind; and he hoped to get a really big scoop. These motives—loyalty, courage, obsessiveness, and ambition—are all plausible, because they are all characteristic of the man.