Donald Kagan, a prominent classical scholar, contentious defender of traditional education and architect of neoconservative foreign policy, died Aug. 6 at a retirement home in Washington. He was 89.

He was a professor emeritus at Yale University, which announced his death but did not give a cause.

Dr. Kagan was a native of Lithuania raised in New York City, who studied ancient Greece in college and was inspired by the “remarkable assumption that the human being is not trivial.” Regarding himself as Greek to his very soul, he wrote several books either entirely or partly about the rise and fall of Athens’s golden age, notably an acclaimed and popular four-volume series on the devastating Peloponnesian War between Athens and Sparta and their respective allies.

“A study of the Peloponnesian War is a source of wisdom about the behavior of human beings under the enormous pressures imposed by war, plague, and civil strife,” he wrote in “The Peloponnesian War” (2003), his one-volume history of the conflict, “and about the potentialities of leadership and the limits within which it must inevitably operate.”

Dr. Kagan expanded upon his belief that the Peloponnesian conflict held vital contemporary lessons in his 1995 book “On the Origins of War and the Preservation of Peace.” With a narrative reaching from ancient Greece and Rome to the two world wars of the 20th century and the Cold War that followed, he determined that some of the most awful carnage could have been avoided had political leaders confronted aggressors early on. He noted the Allies’ hesitation to take on Germany before World War I and World War II. He blamed the 1962 Cuban missile crisis in part on Soviet leader Nikita Khrushchev’s perception that President John F. Kennedy was afraid to use military force.

“The Cuban missile crisis demonstrated that it is not enough for the state that wishes to maintain peace and the status quo to have superior power,” Dr. Kagan wrote. “The crisis came because the more powerful state also had a leader who failed to convince his opponent of his will to use its power for that purpose.”

Through his books, speeches and media commentary, Dr. Kagan became a leading conservative voice in the otherwise liberal field of history, supporting military action abroad and adherence to the Western canon at home.

He backed the wars in Vietnam and Iraq and questioned the patriotism of protesters. He disdained multicultural programs and pushed in vain to establish a special Western Civilization course at Yale. He enraged colleagues when, as dean of Yale College, he told incoming freshmen in 1990 that failure to focus on the West came “at the peril of our students, our country, and of the hopes for a democratic, liberal society emerging throughout the world today.”

“Don should remain a Tory backbencher,” Peter Brooks, chairman of Yale’s comparative literature department, later told The Washington Post. “He’s best as a gadfly, not a dean.”

In 1997, Dr. Kagan joined Richard B. Cheney, Donald H. Rumsfeld and other future George W. Bush administration officials in endorsing the neoconservative Project for a New American Century and its mission statement calling for a “Reaganite policy of military strength and moral clarity.”

Dr. Kagan and his son Frederick Kagan, a scholar of military history, collaborated in 2000 on “While America Sleeps: Self-Delusion, Military Weakness, and the Threat to Peace Today,” which received heightened attention after the 9/11 terrorist attacks.

Bush awarded Dr. Kagan a 2002 National Humanities Medal for his “distinguished scholarship on the glories of ancient Greece” and for teaching generations “the vital legacy of classical civilization.”

Dr. Kagan was born in Kursenai (also known as Kurshan), Lithuania, on May 1, 1932. He was 2 years old when he and his newly widowed mother immigrated to the United States and settled in New York, where as a boy his wary view of humanity was shaped by the antisemitic gangs who menaced him in the Brownsville section of Brooklyn.

The first in his family to attend college, he graduated from Brooklyn College in 1954, received a master’s degree in classics from Brown University the next year and earned a doctorate in history from Ohio State University in 1958.

Like fellow neoconservatives such as Irving Kristol and Norman Podhoretz, he was a Democrat in his youth who turned right in response to the cultural and political upheavals in the 1960s. While teaching at Cornell University, he was enraged by the school’s agreement in 1969 to start a Black studies program after armed protesters occupied a campus building. He compared the decision to British Prime Minister Neville Chamberlain’s appeasement of the Nazis in the 1930s and soon left for Yale.

His wife of 62 years, the former Myrna Dabrusky, died in 2017. In addition to his son Frederick, survivors include another son, Robert Kagan, a contributing columnist at The Post; and two grandchildren.

Dr. Kagan’s model historian was Thucydides, the ancient Greek scholar with whom he shared dark views of human nature, including how among nations power triumphed over morality. He liked to invoke Thucydides’s conclusion that wars were fought out of a combination of fear, self-interest and honor.

“I used to believe that peace was the normal situation for humanity, but the more I looked, the more I saw that peace was very rare,” he told the Yale Alumni Magazine in 2002. “Wars are happening all the time, so I had to ask, ‘Why is there ever peace?’ ”

— Associated Press