John Desmond Patrick Keegan was born on May 15 1934, to a family of Irish Catholic extraction. His father served in the First World War.
At the age of 13 Keegan contracted orthopaedic tuberculosis, which subsequently affected his gait. The long-term effects of his tuberculosis rendered him unfit for military service, and the timing of his birth made him too young for service in World War II, as mentioned in his works as an ironic observation on his profession and interest. The illness also interrupted his education during his teenage years; however, his education included a period at King's College, Taunton, and two years at Wimbledon College, which led to entry to Balliol College, Oxford, in 1953. Following graduation he worked at the American Embassy in London for three years.
In 1960 he was appointed to a lectureship in Military History at the Royal Military Academy Sandhurst, the training establishment for officers of the British Army. Holding the post for 26 years, he became senior lecturer in military history during his tenure. During this period he also held a visiting professorship at Princeton University and was Delmas Distinguished Professor of History at Vassar College.
Leaving the academy in 1986 Keegan joined the Daily Telegraph as a Defence Correspondent and remained with the publication as Defence Editor until his death, also writing for the American conservative website, National Review Online. In 1998 he wrote and presented the BBC's Reith Lectures, entitled War in our World.
Keegan died on 2 August 2012 of natural causes at his home in Kilmington, western England. He is survived by his wife, their two daughters and two sons.