Donald Kagan, a Yale historian whose impassioned teaching and writing about the ancient Greeks inspired generations of scholars as well as Washington strategists, including many of the officials who crafted American foreign policy under Ronald Reagan and George W. Bush, died on Aug. 6 at a retirement home in Washington. He was 89.

His death was confirmed by his son Robert, himself a widely regarded figure in foreign policy circles.

Professor Kagan was considered among the country’s leading historians. His four-volume account of the Peloponnesian War, from 431 B.C. to 404 B.C., was hailed by the critic George Steiner as “the foremost work of history produced in North America in the 20th century.”

He was equally renowned for his classroom style, in which he peppered nuanced readings of ancient texts with references to his beloved New York Yankees and inventive, sometimes comic exercises in class participation, like having students form a hoplite phalanx to demonstrate how Greek soldiers marched into combat.

A strong believer in the timeless virtues of Western civilization and the need for countries to project power in a lawless world, Professor Kagan was often categorized as a conservative. He more or less agreed: He called himself a “Harry Truman Democrat,” but by the late 1960s he had come to believe that the Democratic Party, and much of the academic world, had drifted too far to the left.

He was hard to pin down, though. He disliked Richard Nixon and, more recently, Donald Trump, but he was a fan of Reagan, whose commitment to a strong military and willingness to confront the Soviet Union seemed to him to embody the Greeks’ “mental and intellectual toughness in confronting the human condition.”

Professor Kagan’s politics made him a controversial figure on Yale University’s campus, especially after he became dean of the college in 1989. He was supposed to serve for five years but stepped down in 1992, ostensibly because the faculty rejected his plan to trim the university’s budget.

He had also drawn fire for an article he wrote in which he criticized the humanities faculty as “narrow,” as well as for a plan to create a multidisciplinary course in Western civilization, supported by a $20 million gift from Lee M. Bass, a businessman and alumnus. The planning for the program foundered after Professor Kagan stepped down as dean, and Yale returned the money in 1995.

“There is a bug inside Don, a drive that is not in your average academic,” Paul Kennedy, a fellow Yale historian, said in an interview. “Someone who is a great scholar of Greece and simultaneously this gadfly.”

Professor Kagan was later associated with neoconservatism, though he objected to both the label and the affiliation. He brimmed with opinions on the politics of the day, but remained aloof from Washington and policymaking circles. What connections he did have came mostly through his sons, Robert and Frederick, both of whom played important roles in foreign policy thinking and strategies in the years after the terror attacks of Sept. 11, 2001.

If anything, Professor Kagan often seemed amused at the eagerness of others to define him. Witty and casual, he wore his reputation lightly, never losing touch with his roots in working-class Brooklyn.

“I have led such a lucky, charmed life that I feel like every day is the best day of my life,” he said in a 2012 interview.

Donald Kagan was born on May 1, 1932, in Kursenai, Lithuania. He knew little about his father, Shmuel, who died when he was not yet 2 years old. A few months later his mother, Leah (Benjamin) Kagan, took him and his older sister to New York, where the rest of her family lived.

The Kagans settled in the Brownsville neighborhood of Brooklyn, which at the time had a bustling Jewish population. While his mother worked in her family’s factory, which made smoking pipes, and later ran her own corset shop, Donald was left free to roam the streets, playing with his friends and keeping an eye out for bullies — an experience that, he later said, shaped his view of the world as a fundamentally lawless and violent place, where standing up for yourself and your allies took courage.

He played football at Thomas Jefferson High School, an institution that educated generations of Jewish immigrants and their children. It was there that he first took an interest in history, and where he met Myrna Dabrusky. They married in 1954.

Mrs. Kagan, who taught elementary school and wrote two books about the history of New Haven, Conn., died in 2017. Along with his sons, Professor Kagan is survived by two grandchildren.

He attended Brooklyn College and later received a master’s degree in history from Brown University and, in 1958, a doctorate from Ohio State University. He taught briefly at Pennsylvania State University and traveled to Greece on a Fulbright, his wife and newborn son Robert in tow, before settling at Cornell University.

Professor Kagan fell in love with Cornell, especially the collegiality of its faculty. But in 1969, when armed Black students took over an administration building, demanding the creation of an Africana studies center and amnesty for fellow students who had been disciplined for an earlier protest, the university’s decision to negotiate with them struck him as a capitulation to violence. Months later he decamped for Yale. The crisis at Cornell was, he later said, the worst experience in his life.

Though he at first admired Kingman Brewster, Yale’s president, for his stand against campus radicalism, in 1974 Professor Kagan publicly criticized him after the university canceled a speech by William Shockley, a Stanford physicist and Nobel Prize winner who believed that Black people were genetically inferior. Professor Kagan strongly disagreed with Shockley’s views, but he believed the university should be exposing students to challenging points of view.

In response to that criticism, Mr. Brewster asked the historian C. Vann Woodward to write a report about campus speech, and later adopted many of its proposals that lined up with Professor Kagan’s views.

Professor Kagan’s passion for ancient Greece informed another of his great loves: sports. He liked to say that one root of his contrarian nature was that as a child in 1930s Brooklyn, he was a Yankees fan in a sea of Dodgers caps. Among his greatest moments, he said, was the year Yale asked him to serve as acting athletic director, a job he relished even as he continued to teach history.

He saw baseball as a Homeric allegory, one in which a hero — the batter — ventures from home and must overcome unforeseen challenges in order to return. That view set up one of his most celebrated articles: a withering review in The Public Interest of the columnist George Will’s book “Men at Work: The Craft of Baseball” (1990).

“This is the fantasy of a smart, skinny kid who desperately wants to believe that brains count more than the speed, power and reckless courage of the big guys who can play,” Professor Kagan wrote.

The contretemps over his time as dean made Professor Kagan something of an academic celebrity. After stepping down, he continued to pursue the life of a public intellectual, publishing essays and reviews in The Wall Street Journal, The New York Times and The Weekly Standard.

In 1997 he signed the founding principles of the Project for the New American Century, a think tank of which his son Robert was a creator that called for the country to reassert its power in the post-Cold War world. In 2000 he and his son Frederick published “While America Sleeps,” which likewise criticized what they saw as a dangerous drift in U.S. foreign policy.

Otherwise, his influence on policymaking was indirect and passive.

“He wasn’t one of those people from the Harvard Kennedy School who traipsed down to Washington to converse with policymakers,” said Eric Edelman, who studied with Professor Kagan and later served in a number of senior national security roles.

Many of his students, like Dr. Edelman, went on to fill top jobs in the federal government, and his books proved influential among foreign policy hawks — especially “On the Origins of War and the Preservation of Peace” (1994), which analyzed five crises and conflicts, ancient and modern, and argued that leaders need to show not just power but also courage in the face of challengers.

Professor Kagan received a National Humanities Medal in 2002. Three years later he delivered the annual Jefferson Lecture, sponsored by the National Endowment for the Humanities, in which he praised the study of history but warned that it was succumbing to the influence of postmodern relativism.

He sounded a similar alarm in his final lecture at Yale, in 2013. Liberal education, he said, was failing to provide students with a common set of values and the tools to make sense of the world.

“I find a kind of cultural void and ignorance of the past, a sense of rootlessness and aimlessness, as though not only the students but the whole world was born yesterday,” he said.

When he finished, despite his pessimistic message, he received a standing ovation.