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 August 6 at a retirement home in Washington. He was 89.

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 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 exercises in class preparation. He once had students form a hoplite phalanx to demonstrate how Greek soldiers marched into combat.

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.