Zara Steiner, a historian whose magisterial books on 20th-century diplomacy were considered authoritative studies of Europe from World War I to World War II, died Feb. 13 at her home in Cambridge, England. She was 91.

The cause was pneumonia, said her son, David Steiner.

Dr. Steiner was a U.S.-born scholar who spent most of her career in England, including a long association with the University of Cambridge. She was part of an intellectual power couple with her husband, the literary critic and author George Steiner, who died Feb. 3.

Early in her academic life, Dr. Steiner published studies on the U.S. Foreign Service, but after she and her husband settled in Cambridge in the early 1960s, she focused her scholarly attention on Europe. She became a leading figure at New Hall (now called Murray Edwards College), a women’s college within the University of Cambridge.

In 1969, Dr. Steiner published a well-regarded history of British foreign policy from 1898 to 1914, followed in 1977 by “Britain and the Origins of the First World War.”

Perhaps her most acclaimed work came late in her career, with two volumes in the Oxford History of Modern Europe series, which began in 1954 with A.J.P. Taylor’s “The Struggle for Mastery in Europe 1848–1918.” Dr. Steiner is the only female historian to have contributed to the series, now containing 15 titles.

Her 2005 book, “The Lights That Failed: European International History, 1919–1933,” took up where Taylor left off. In 2010, she published “The Triumph of the Dark: European International History, 1933–1939.”

Each book was about 1,000 pages long and demonstrated an encyclopedic knowledge of European history, economics and diplomacy. The second volume, in particular, depicted a Europe descending into totalitarianism, with the menacing rise of Hitler in Germany, Mussolini in Italy and Stalin in the Soviet Union.

She described Hitler as “an opportunist who knew where he was going” and portrayed Stalin as a cruel tyrant whose purges of more than 1 million people in his country in the 1930s grew out of his “fierce determination to establish his control over all men and institutions that might threaten his monopoly of power.”

“Taken together,” the historian Vernon Bogdanor wrote in Britain’s New Statesman magazine, “the two volumes are a remarkable achievement of conscientious scholarship.”

Another historian, Piers Brendon, writing for Britain’s Independent newspaper, called “The Triumph of the Dark” “austerely academic,” with “few telling anecdotes, colourful vignettes or vivid character sketches.”

Nevertheless, he pronounced the 1,138-page book “a terrifically good read.”

Zara Alice Shakow was born Nov. 6, 1928, in New York City. Her father operated an outfitting shop that sold supplies to polar explorers and other adventurers. Her mother was a homemaker.

Dr. Steiner was a 1948 graduate of Swarthmore College in Pennsylvania, then received bachelor’s and master’s degrees from the University of Oxford in 1950 and 1954, respectively. She received a doctorate in history from Harvard University in 1957. In 2007, she was elected a fellow of the British Academy.

She and George Steiner were married in 1955. Survivors include a son, David Steiner, of Baltimore, who is a professor at Johns Hopkins University and the executive director of the university’s Institute for Education Policy; a daughter, Deborah Tarn Steiner, of Princeton, N.J., who is a classics professor at Columbia University; and two granddaughters.

In “The Triumph of the Dark,” Dr. Steiner described the period leading up to World War II as a time of “few heroes, two evil Titans and an assortment of villains and knaves. I have not enjoyed their company.”