Ronald Steel, a historian and foreign policy expert best known for his monumental, prizewinning biography of one of the 20th century’s most influential political commentators, Walter Lippmann, and who also wrote a revisionist study of Robert F. Kennedy that took some sheen off the family’s Camelot myth, died May 7 at a nursing home in Washington. He was 92.

He had a progressive cognitive impairment, but the immediate cause was unknown, said his physician and friend, Michael Newman.

Mr. Steel, a high-profile public intellectual, was affiliated with think tanks and universities throughout his career and was a frequent contributor to the New York Review of Books, the Atlantic Monthly and other publications. A former Foreign Service officer, he established his reputation with two early books on international relations, including “Pax Americana” (1967).

In that book, Mr. Steel argued that a postwar obsession with the communist threat led to a counterproductive, financially draining U.S. military buildup. An inevitable consequence of such a foreign policy, he said, was to engage in military action rather than seek diplomatic or economic solutions to international conflicts.

Historian Henry Steele Commager, writing in the New York Times, pronounced “Pax Americana” the “most ardent and, to my mind, the most persuasive critique of American foreign policy over the last twenty years that has yet appeared.”

Mr. Steel later planned to write about Lippmann and the Cold War — a phrase first popularized by Lippmann after World War II to describe the ideological chess match between the Soviet Union and Western democracies.

The project soon evolved into a full-scale biography of the prolific author and two-time Pulitzer Prize winner whose syndicated column appeared in hundreds of newspapers.

Lippmann was a monumental figure — “without doubt the nation’s greatest journalist,” in Mr. Steel’s words — whose ideas helped define the thinking of the political establishment for decades. Mr. Steel interviewed him at length before Lippmann’s death in 1974.

“I had no idea what I was getting into,” Mr. Steel told The Washington Post in 1980. “He’d been writing three columns a week since 1931. He’d written 22 books. . . . He’d known everybody and been a critical and articulate observer of this country since 1912.”