British historian Hugh Thomas in 2008. (FRANCOIS MORI/AP)

Hugh Thomas, a historian who wrote sweeping accounts of rebellion, conquest and struggle, particularly about parts of the world touched by the Spanish empire, and who was an adviser to British Prime Minister Margaret Thatcher during the Falklands War, died May 6 in London. He was 85.

His death was first reported in British news accounts. His family said he had a stroke.

Mr. Thomas wrote two novels before gaining acclaim as a historian in 1961 with “The Spanish Civil War.” Memories were still fresh about the war, which lasted from 1936 to 1939, and Mr. Thomas’s narrative flair and political analysis gave his book the quality of a modern-day epic.

“The Spanish Civil War exceeded in ferocity most wars between nations,” Mr. Thomas wrote. “It was, for the Western World at least, a most passionate war.”

In popular memory, the war seemed to be a dramatic enactment of a clear moral choice between a Marxist-inspired democratic left and the military might of a repressive fascist-aligned regime. Mr. Thomas realized that perception was, at best, incomplete.

“The Spanish War,” he wrote, “appeared as a ‘just war,’ as civil wars do to intellectuals, since they lack the apparent vulgarity of national conflicts. . . . It looked, at least at first, when all the parties of the Left seemed to be cooperating, as the great moment of hope for an entire generation.”

In the end, the left-leaning groups were defeated by the nationalist forces of Gen. Francisco Franco, who went on to rule Spain as an autocrat for almost 40 years. Mr. Thomas’s book portrayed the vast complexity of the Spanish Civil War, with its background of feudalism and religious fervor, but it was also seen as the chronicle of a noble but failed cause.

For that reason, his book was banned under Franco’s regime. Copies were smuggled across the border, and Spaniards caught in possession of the book sometimes went to prison. It wasn’t until after Franco’s death in 1975 that Mr. Thomas’s history of the civil war could be freely distributed in Spain.

“The Spanish Civil War” was the first of more than a dozen large-scale historical studies by Mr. Thomas, including books on European history and the origins of the Cold War.

His 1997 book “The Slave Trade” was “the most comprehensive account of the Atlantic slave trade ever written,” UCLA scholar Robert B. Edgerton wrote in the National Review.

Mr. Thomas was best known for a series of books about the Spanish-speaking world, including a monumental, 1,700-page history of Cuba, published in 1971. (When Mr. Thomas turned to “A History of the World” in 1979, he covered the subject in a mere 700 pages.)

In the 1990s, he published a book on the Spanish conquest of Mexico, followed by his ambitious “Spanish Empire” trilogy, the final volume of which, “World Without End,” appeared in Britain in 2014 and in the United States a year later. His overtly popular style of writing, filled with vivid characters and action, led to sales in the millions.

He was 72 when his 700-page “Rivers of Gold: The Rise of the Spanish Empire, From Columbus to Magellan,” was published.

“A book the size of ‘Rivers of Gold’ would be an astonishing work by any author,” historian Paul Kennedy wrote in the New York Times in 2004, “yet its publication simply affirms Hugh Thomas’s record as one of the most productive and wide-ranging historians of modern times.”

Hugh Thomas was born Oct. 21, 1931, in Windsor, England, outside London. His father was a colonial administrator in Africa.

Mr. Thomas was a 1952 graduate of the University of Cambridge and worked in the British Foreign Office before deciding to become in independent writer in 1957. He taught at Britain’s University of Reading from 1966 to 1976.

In the 1950s, Mr. Thomas ran unsuccessfully as a Labour Party candidate for Parliament. In the ensuing years, his political inclinations grew progressively more conservative. (His history of Cuba was sometimes criticized for being too harsh on the communist economic policies of Fidel Castro.)

By the late 1970s, Mr. Thomas had drifted toward the Conservative Party, whose leader, Thatcher, was elected British prime minister in 1979. Because of his expertise on the history of the Spanish-speaking world, Mr. Thomas was a close adviser to Thatcher during the 1982 British war with Argentina in the Falkland Islands.

She named him to a peerage in 1981, and he served in the House of Lords as Baron Thomas of Swynnerton.

Beginning in 1979, Mr. Thomas led the Center for Policy Studies, a conservative London think tank. His strong support of British participation in the European Union was often at odds with prevailing conservative orthodoxy and ultimately led to his resignation from the center in 1991.

Survivors include his wife of 55 years, the former Vanessa Jebb, and three children.

On the whole, Mr. Thomas said the Spanish conquest of the New World was inspired by a “desire for some kind of glory, to cut some kind of dash in the world.”

Yet the Spanish also had a sense of self-reflection that was not demonstrated by any other colonial power. “In no other imperial history,” Mr. Thomas told NPR in 2011, “the British or the Roman or the French or the Chinese, do we see such a discussion of this nature.”

The Spanish may have been “brutal and sometimes contradictory, sometimes foolish,” Mr. Thomas said, yet they also questioned “whether they had any right to be in the New World.”