Honorable men, as they were called in the golden age of chivalry, were rich enough to outfit a squadron of knights and brutal enough to lead them into battle, often culminating in the killing and plundering of civilian populations. The code of chivalry defined honor in ways that are familiar to people today — as honesty, loyalty, courage — and in ways that are not. It was honorable, for example, to show mercy to a defeated enemy, but only if the enemy was a social equal.

There was no dishonor in slaughtering commoners.

Maurice H. Keen, a historian who presented that unvarnished view of the medieval nobility in his book “Chivalry,” was one of a small group of scholars in the 1980s who re-examined the record of the chivalric knights, long portrayed in romantic literature as do-gooders, and who found it — with all due respect to Thomas Malory and Walter Scott — incomplete.

There were many do-gooders and brave fellows, no doubt. And the chivalric code did moderate and civilize men’s behavior, especially toward women of equal status. But Keen, who died on Sept. 11, argued that chivalry was mainly a “cult of martial virtues” for men and about men, charting a path to glory, honor and wealth. From about 1150 to 1500, he wrote, obeying its code was the only way for aristocrats to move higher on the social ladder and the only way for commoners to reach the first rung.

“Chivalry, in effect, was a protection plan for the warrior class,” said Richard P. Abels, chairman of the history department at the U.S. Naval Academy in Annapolis, Md., and a professor of medieval history. It was a system, he said, for shielding knights and their families (and no one else) from the worst excesses of war.

Keen, whose books on medieval history have been taught at the Naval Academy as well as at the U.S. Military Academy in West Point, N.Y., died in Oxford, England, after several years of declining health. He was 78. His death was announced by Balliol College, where he was named an emeritus fellow in 2000 after four decades as a teacher and administrator.

Keen wrote or edited almost a dozen books on the Middle Ages. But “Chivalry,” published in 1984, was his most influential because it so sharply redefined medieval court life, challenging a view that had been dominant for hundreds of years.

In that view, chivalry was a code of behavior that emerged in the 12th century as a kind of self-improvement guide for men — who spent a lot of time killing — seeking to familiarize themselves with Christian values and humane principles and become gentlemen. It promoted fair fighting, for example, and the protection of women and children.

“Keen said that that was true enough, but only part of the picture,” said Clifford Rogers, a professor of history at West Point. “His great insight was that chivalry was synonymous with the law of war — an international body of law agreed upon by the aristocratic classes across just about all of Europe, from the 12th to the 15th centuries.”

Keen’s book was among the first to “cut through all the stuff about courtly love and show that chivalry was an important part of the social history of warfare,” said C. Stephen Jaeger, a medieval historian and professor emeritus at the University of Illinois.