Robert L. Beisner, a prizewinning diplomatic historian, in 2004. (Jeff Watts/American University)

Robert L. Beisner, a prizewinning diplomatic historian best known for his exhaustive biography of former secretary of state Dean Acheson, one of the towering statesmen of his generation, died Jan. 31 at a hospital in Washington. He was 81.

The cause was complications from pneumonia, said his stepdaughter, Signe Williamson.

The urbane, imposing, wasp-tongued Acheson, with his trademark thick eyebrows and well-tended mustache, wielded enormous influence over foreign and domestic policy during several administrations before serving as President Harry S. Truman’s undersecretary of state and, eventually, as the country’s top diplomat from 1949 to 1953.

In his sundry roles, and fortified by his blend of pragmatism and liberal idealism, Acheson worked on the establishment of the World Bank, and helped shape the North Atlantic Treaty Organization and the post-World War II reconstruction of Europe known as the Marshall Plan.

He oversaw diplomatic negotiations during the Korean War and spun a web of military and political alliances to contain the ambitions of communism, from West Germany to Taiwan, setting the course for the United States’ global ambitions. With not entirely undue immodesty, he called his memoir, published two years before his death in 1971, “Present at the Creation.”

Dr. Beisner, a past chairman of American University’s history department and president of the Society for Historians of American Foreign Relations, was not the first or last scholar to assess Acheson’s daunting legacy. Gaddis Smith’s “Dean Acheson” (1972), David S. McLellan’s “Dean Acheson: The State Department Years” (1976), Douglas Brinkley’s “Dean Acheson: The Cold War Years, 1953-71” (1992) and James Chace’s “Acheson: The Secretary of State Who Created the American World” (1998) were among the volumes that studied Acheson’s dominance over Cold War foreign affairs.

But Dr. Beisner’s 800-page “Dean Acheson: A Life in the Cold War,” which came out in 2006, was well regarded for its attempt to take the full measure of the man, who was vilified and admired in equal measure.

Reviewing the book in the New York Times, former secretary of state Henry Kissinger called it a “sweeping and thoughtful account of Acheson’s tenure” that “does not always capture the vividness of his personality, which emerges too much as a list of eccentricities.” Journalist and author Walter Isaacson, also writing in the Times, praised Dr. Beisner for a “a solidly researched and balanced tome” that was not “rollicking and witty [but] far more reliable and accurate than Acheson’s martini-lubricated memories.”

Robert Lee Beisner was born in Lexington, Neb., on March 8, 1936; his father owned a car and tractor dealership, and his mother was a homemaker. He attended Nebraska’s Hastings College for two years, then left for the University of Chicago, where he received a master’s degree in 1960 and a doctorate in 1965, both in history.

He joined the American University faculty that year and stepped down in 1998, quipping at the time to a Chicago alumni publication, “Retirement is great: For the first time since I was about 6, I enjoy Sunday evenings!”

His first book, “Twelve Against Empire: The Anti-Imperialists, 1898-1900” (1968), won honors from the American Historical Association and the Society of American Historians. The volume, focused on the rise of American political and military commitments in the Far East and the Caribbean, gingerly hinted at parallels to the roiling protests over the Vietnam War.

Dr. Beisner also wrote “From the Old Diplomacy to the New, 1865-1900” (1975). He compiled a monumental study, “American Foreign Relations Since 1600: A Guide to the Literature” (2003), for the Society for Historians of American Foreign Relations.

His first marriage, to the former Mary Brinton Stone, ended in divorce. In 1976, he married Valerie French, who became chairwoman of American University’s history department and associate dean of the College of Arts and Sciences. She died in 2011.

Survivors include two children from his first marriage, John Beisner of Linden, Va., and Katharine Beisner of Austin; two stepchildren, Signe Williamson of Beaufort, S.C., and John Allen of Washington; and six grandchildren.

In a 2006 interview with Oxford University Press, Dr. Beisner said he doubted that secretaries of state — including then-chief diplomat Condoleezza Rice — had absorbed lessons from Acheson.

He said he was “struck by how many presidents and secretaries of state have historically spent a year or so acting clumsily in the international arena before coming to terms with it before — usually — improving their performance.

“That is partly because of the partisanship of our politics and nature of our constitutional system, for each new administration wants to take credit for improving on or even repudiating its predecessor,” he added. “If I were forced to suggest what Secretary Rice should pay special attention to, it would be the Achesonian view that only by accepting restraints on its behavior vis à vis allies can Washington successfully lead its more dependent allies. But I suspect she already knows that.”