In 1986, the historian John Elliott published a massive biography of a 17th-century Spanish statesman, Gaspar de Guzmán, the count-duke of Olivares. John, who has died aged 91, had already published numerous acclaimed books although his first, Nibble the Squirrel, written for children in 1946, is a lesser-known part of the canon.

Although already a toweringly influential historian in Britain, Spain and the US, with his 1963 work Imperial Spain, 1469-1716 still regarded as the go-to introduction to the Habsburg period, it was the bombshell of the 1986 biography that put Olivares on the map and was probably instrumental in John’s elevation in 1990 to the regius chair of modern history at Oxford.

The volume was hailed by Raymond Carr in the New York Review of Books as “what must rank as the finest biography ever written on a Spanish statesman”. After it, no historian could remain unaware of the all-powerful, if ultimately unsuccessful, factotum of Spain’s ineffectual Philip IV, a king so concerned with the arts and his innumerable mistresses that he left the administration of the country’s sprawling, ramshackle empire to Olivares. In shedding light on the great adversary of Cardinal Richelieu, John rendered comprehensible the central power struggle of 17th-century Europe, a contest in which the Spaniard was the loser.

John’s pursuit of Olivares was lifelong, a hint of which was the brilliant portrait in his first major work, The Revolt of the Catalans: A Study in the Decline of Spain, 1598-1640 (1963). The fascination had begun when, as a Cambridge undergraduate, he first saw the Diego Velázquez portrait of Olivares in the Prado museum in Madrid. It was to be an awesomely difficult task. Most of Olivares’ papers that survived his own cavalier approach to record-keeping were lost in fires in the 18th century. In order to assemble the remnants, John spent a quarter of a century searching in 16 private and public archives in Spain alone, as well as in eight other countries. The triumphant result was the stunningly researched and vividly written biography.

Imperial Spain was the first of a series of path-breaking books on the imperial struggles between France and Spain that, in turn, opened up a new field of world history. Another distinguished historian of Spain, Victor Kiernan, said of it: “General readers coming more or less fresh to the subject will be grateful for this masterly introduction to it and … [specialists] who have struggled with it already will receive much fresh light.”

John himself commented in the preface that one of his aims in writing the history of Habsburg Spain was to indicate “how much remains to be done before we can confidently claim to have found the answers”.

The book was, in his own words, a work of “interpretative synthesis” rather than conventional narrative, a skill in which he excelled. Along the way, the book sparkled with perceptive insights. Take for example this brilliant summary of Spain’s agricultural limitations: “A dry, barren, impoverished land: 10 per cent of its soil bare rock; 35 per cent poor and unproductive; 45 per cent moderately fertile; 10 per cent rich.”

His explanation of the popular appeal of the antisemitism that underlay the work of the Inquisition was equally memorable, illustrating how the poor could seek consolation in their “purity” of blood unlike the aristocrats who had often intermarried with wealthy families of converted Jews. In this and other books could be discerned John’s concern with parallels between the loss of empire and national decline in the Spain of the 1620s and Britain in the 1950s, what Kiernan called “a condition of national imbecility such as few nations have ever sunk to”.

In 2012, he published his memoir History in the Making, a commentary on the changing nature of historical writing in his lifetime, together with reflections on his own career, what he modestly called “the testimony of a historian who has tried to understand”.

That was a worthy vocation for a historian who eschewed theory, wrote accessibly and believed in the importance of human agency in shaping great historical events. He defined the key to the writing of good history as “the ability to enter imaginatively into the life of a society remote in time or place, and produce a plausible explanation of why its inhabitants thought and behaved as they did”.

Born in Reading, to Janet (nee Payne) and Thomas Elliott, a headmaster, John went to Eton college, but entirely lacked the arrogance and entitled bluster of better-known products of the school.

After graduating in history from Trinity College, Cambridge, in 1952, he had a distinguished career as a lecturer there (1957-67) and five years as professor at King’s College London before moving in 1973 to what he called the “scholar’s paradise” at Princeton, where he stayed for nearly 20 years.

I got to know John shortly after his return to Britain to take up the regius chair at Oxford. Previously, my knowledge of him came from his books, including The Revolt of the Catalans, which had given him hero status in Catalonia. There, over the years, as in Spain, he was showered with prestigious prizes.

To learn Catalan, he had lodged with a family in Barcelona. “Before my stay was over I was even dreaming in Catalan,” he wrote later. He had also acquired a sense of the repression of Catalans under the Franco dictatorship. A striking experience came one day when, innocently speaking in Catalan, he asked a member of the Policía Nacional for directions. The furious policeman snapped: “Speak the language of the empire!”

A more nuanced awareness of the history of Catalonia’s relationship with Spain came with studying with the Catalan historian Jaume Vicens i Vives, whose influence meant that John avoided falling prey to the theme of “chosen nation syndrome or innocent victim syndrome”. Thus, his most recent book, Scots and Catalans: Union and Disunion (2018), which made sage criticisms of the most recent Catalan independence movement, did not, inevitably, please the more radical Catalanistas.

At Oxford, John set up a small taskforce to work on a not entirely successful project to reform the teaching of history at the university. As a member of his team, I got to know the tall and rather gaunt figure, invariably quiet and courteous.

He was a reserved and abstemious man, so much so that it was assumed that he would live long beyond his 91 years. In his work and his life, the watchwords were grace and humility.

He was knighted in 1994.

His wife, Oonah (nee Butler), whom he married in 1958, survives him.