

His next work was Last Battle The Classic History of the Battle for Berlin (1966), about the Battle of Berlin. Published in 1959 it was an instant success. In 1956 he began to write down his World War II notes for The Longest Day, which tells the story of the invasion of Normandy.

He began compiling information and conducting over 1000 interviews as he gathered stories from both the Allies and the Germans, as well as the French civilians. On a trip to Normandy in 1949 Ryan became interested in telling a more complete story of D-Day than had been produced to date. He married Kathryn Morgan (1925–1993), a novelist, and became a naturalized citizen of the United States in 1951. This was followed by work for other magazines, including Collier's Weekly and Reader's Digest. Ryan emigrated to the United States in 1947 to work for Time magazine, where he reported on the postwar tests of atomic weapons carried out by the United States in the Pacific. He transferred to the Pacific theater in 1945, and then to Jerusalem in 1946.

He initially covered the air war in Europe during WW II, flew along on fourteen bombing missions with the Eighth Air Force and Ninth Air Force United States Army Air Forces (USAAF), then joined General Patton's Third Third Army and covered its actions until the end of the European war. After finishing his education Ryan moved to London in 1940, and became a war correspondent for ''The Daily Telegraph'' in 1941.
