In June 1943, he was sent to the European Theater of Operations, when he was requested by his another old superior from Infantry School, General Omar Bradley. Ryan was attached to the headquarters, Twelfth United States Army Group as assistant chief of staff for military plans and training. He served in this capacity during the Battle of Normandy, Ardennes Campaign, Rhineland Campaign and Western Allied invasion of Germany and was promoted to the temporary rank of brigadier general on January 7, 1945.