John Keegan appeared at the Pentagon Book Store on June 4, 1996. (LARRY MORRIS/THE WASHINGTON POST)

John Keegan, a British military historian whose groundbreaking book “The Face of Battle” cast a fresh look at warfare, capturing the fears, anxiety and heroism of the front-line soldier, died Aug. 2 at his home in Kilmington, England. He was 78.

The Telegraph newspaper in London, for which he had been an editor and writer, announced his death but did not disclose the cause.

Mr. Keegan was a writer of exceptional grace whose more than 20 books spanned the history of warfare from Alexander the Great to the 21st-century U.S. incursion in Iraq. In the New York Times Book Review in 2009, Civil War historian James M. McPherson called him “our generation’s foremost military historian.”

The work that established Mr. Keegan’s reputation was “The Face of Battle” (1976), which examined warfare from the perspective of soldiers taking part in three historical European battles: Agincourt in 1415; Waterloo in 1815; and the 1916 Battle of the Somme, during World War I.

In the book, Mr. Keegan made it clear that his approach was a departure from conventional historical writing, “with its reduction of soldiers to pawns, its discontinuous rhythm, its conventional imagery, its selective incident and its high focus on leadership.”

Instead, he described the conditions at the battlefront with a visceral realism, depicting the deafening noise, blinding smoke, slippery gore, confusion, stench and fear that accompanied soldiers in war.

All battles, regardless of their era, Mr. Keegan argued, were fought by soldiers “struggling to reconcile their instinct for self-preservation, their sense of honor and the achievement of some aim over which other men are ready to kill them.”

He was particularly eloquent in writing about the brutality of the first day of the Battle of the Somme, on July 1, 1916: “In all, the British had lost about 60,000, of whom 21,000 had been killed, most in the first hour of the attack, perhaps the first minutes.”

The conditions of the battle were so abhorrent, he wrote, with “long docile lines of young men, shoddily uniformed, heavily burdened, numbered about their necks, plodding forward across a featureless landscape to their own extermination,” that they could be compared only to the Nazi concentration camps of World War II.

“Accounts of the Somme,” Mr. Keegan wrote, “produce in readers and audiences much the same range of emotions as do descriptions of the running of Auschwitz — guilty fascination, incredulity, horror, disgust, pity and anger.”

Historian Neal Ascherson, writing in the New York Review of Books, said that “The Face of Battle” had “no counterpart in the literature of war.”

British historian J.H. Plumb, in the New York Times Book Review, pronounced it “a brilliant achievement” that was “as much about the nature of man as of battle.”

Mr. Keegan went on to write other influential books exploring the nuances of warfare, including “Six Armies in Normandy” (1982), about the D-Day Invasion of France during World War II, and “The Mask of Command” (1987), which concluded that the best military commanders projected strength through a strong sense of theatricality that inspired their soldiers to follow them into battle.

In his 1996 book, “Fields of Battle: The Wars of North America,” Mr. Keegan described the military and cultural significance of landscape and a continent-wide network of forts in battles throughout the United States and Canada.

“Keegan reveals geography as destiny,” historian T.H. Watkins wrote in The Washington Post, “the beautifully described coastlines, rivers, valleys, forests, hills, hollows, mountain ranges and wide open spaces of the continent becoming every bit as important to the story as the generals and the armies who bled and died within it.”

John Desmond Patrick Keegan was born May 15, 1934, in London. His father was an inspector of schools.

During the World War II bombing blitz of London, as Mr. Keegan wrote in “Six Armies in Normandy,” he was “whisked from London at the first wail of the sirens to a green and remote corner of the west of England and kept there until the echo of the last shot fired was drowned in the sighs of the world’s relief in August 1945.”

While there, he met troops from the United States and other allied countries, heard the steady drone of military airplanes and could identify them by shape and sound.

At 13, Mr. Keegan was stricken with a form of tuberculosis that infected his hip and required years of hospital treatment and rehabilitation. He walked with a pronounced limp throughout his life.

After receiving a degree in history from the University of Oxford, Mr. Keegan came to the United States in 1957 to explore Civil War battle sites. Back in London, he prepared political reports for the U.S. Embassy for two years before becoming a lecturer at the British Royal Military Academy at Sandhurst from 1960 to 1986.

He never served in the military himself and didn’t see a live battle until he was dispatched to cover the Lebanese civil war in 1984 by the Daily Telegraph. He later told Newsweek “how physically disgusting battlefields are . . . like being in a municipal garbage dump.”

In 1986, Mr. Keegan resigned his professorship at Sandhurst to become defense editor of the Telegraph, where he worked for more than 20 years. He was in demand as a lecturer throughout Britain, the United States and Canada and was a visiting professor at Princeton University and Vassar College, among other institutions. He briefed President Bill Clinton on the 50th anniversary of D-Day in 1994 and was knighted in 2000.

Mr. Keegan was not immune to criticism. Reviewers found his 2009 book, “The American Civil War,” riddled with errors, and some historians said he slighted the political ramifications of warfare. Although he once described himself as “95 percent pacifist,” he defended U.S. involvement in Vietnam and later in Iraq.

Near the end of his life, he wrote a column about rural village life in the English countryside.

Survivors include his wife of 52 years, biographer Susanne Everett; and four children.

If Mr. Keegan’s views can be summarized, he was a scholar who admired soldiers but deplored war. In his 1993 book, “A History of Warfare,” he admired the example of earlier, presumably more “primitive” people, who did their utmost to avoid conflict and were “devotees . . . of restraint, diplomacy and negotiation.”

With modern weapons threatening to destroy civilization, Mr. Keegan wrote, it was imperative not to forget those lessons from the distant past.

“Unless we unlearn the habits we have taught ourselves,” he wrote, “we shall not survive.”