Cornelius Ryan was an Irish-American journalist and author known mainly for writing popular military history. He was especially known for his histories of World War II events: The Longest Day: 6 June 1944 D-Day (1959), The Last Battle (1966), and A Bridge Too Far (1974).

The Longest Day is a 1959 book by Cornelius Ryan telling the story of D-Day, the first day of the World War II invasion of Normandy. It details the coup de main operation by gliderborne troops, which captured the Caen canal and Orne river bridges before the main assault on the Normandy beaches. It sold tens of millions of copies in eighteen different languages. It is based on interviews with a cross-section of participants, including U.S., Canadian, British, French and German officers and civilians.