Gaddis Smith, a Yale historian who taught generations of policymakers and politicians the finer points of U.S. foreign policy and who, in the 1970s and ’80s, helped the university navigate between its desire to modernize and the powerful alumni who opposed such change, died on Friday at his home in New Haven, Conn. He was 89.

His son, Edgar Smith, said the cause was progressive supranuclear palsy, a brain disorder.

Dr. Smith was a Yale institution. He arrived on campus as a freshman in 1950, received his doctorate from the university in 1961, and, aside from a short teaching stint at Duke, never left.

He was one of the few professors whose classes were considered informal requirements by undergraduates regardless of major. Shy in small groups, he blossomed in front of the hundreds of students who packed his lectures.

Among his many fans were the actress Jodie Foster, who studied literature at Yale in the 1980s but considered his course on American foreign policy one of her favorites, and Samantha Power, the administrator of the United States Agency for International Development, who wrote her senior thesis under his direction.

“Because he was such a compelling and lively teacher, legions of students crammed into large lecture halls every year to learn from him,” Ms. Power wrote in an email. “Generations of practitioners, teachers and critics of U.S. foreign policy learned how to question received wisdom and old habits in pursuit of a more impactful and humane course.”

He taught both Senator John Kerry and President George W. Bush. When the two of them ran against each other for the White House in 2004, reporters called Dr. Smith for comment.

He was sufficiently circumspect not to say much, though he did say that Mr. Bush had earned C’s and seemed more interested in social life on campus than the more serious, ambitious Mr. Kerry.