John Keegan, an Englishman widely considered to be the pre-eminent military historian of his era and the author of more than 20 books, including the masterwork “The Face of Battle,” died Thursday at his home in Kilmington, England. He was 78.

His death was announced in The Telegraph, where he had served as the military affairs editor. No cause of death was given, though Con Coughlin, the paper’s executive foreign editor, said in an e-mail that Mr. Keegan had died after a long illness.

Mr. Keegan never served in the military. At 13, he contracted orthopedic tuberculosis and spent the next nine years being treated for it, five of them in a hospital, where he used the time to learn Latin and Greek from a chaplain. As he acknowledged in the introduction to “The Face of Battle,” he had “not been in a battle, nor near one, nor heard one from afar, nor seen the aftermath.”

But he said he learned in 1984 “how physically disgusting battlefields are” and “what it feels like to be frightened” when The Telegraph sent him to Beirut, Lebanon, to write about the civil war there.