In 1954, Denis Mack Smith, who has died aged 97, produced his first book – and his masterpiece – Cavour and Garibaldi 1860: A Study in Political Conflict. Written with verve and style, it unpicks a crucial year in Italian history, undercutting a series of accepted truths concerning Italy’s unification and, in Jonathan Steinberg’s words, telling Italians “what they did not want to hear”.

For Denis, far from being a collaboration by great men, the Risorgimento had been riven by conflict, contradictions and debates over strategy. And while previous interpretations had depicted Giuseppe Garibaldi as a romantic and wild revolutionary, and Count Camillo di Cavour as a brilliant and serious tactician, Denis argued that things had often been the other way around. He had unwittingly walked straight into a fierce debate in Italy between followers of Antonio Gramsci, the Communist leader and theorist who died after imprisonment by Mussolini, with their conviction that the Risorgimento had been a failed revolution, and liberal historians, who defended the gains of the unification period.

His analysis fitted perfectly, at least on the surface, with the Gramscians, although he admitted that, at that point, he had not read any Gramsci. Not a man of the left but an anti-fascist liberal, he was often bundled into the left camp in Italy, where history has always been intensely politicised.

Denis was a rarity, a true populariser of history, who wrote in a clear, readable and engaging style. His writing was full of quotable lines. He wrote of Cavour that he had “managed to persuade people to back a revolution on the excuse that this was the way to prevent a revolution”.

He was also a master of the use of a crucial anecdote and the minor but telling detail. The public adored his output and his History of Italy (1959) became the bestselling text written by an academic of all time in Italy – shifting something like 200,000 copies. He was reluctant to have it translated into Italian, and was only persuaded to do so by the publisher Vito Laterza, who “didn’t change a line”. The book was also given out to subscribers of a Communist party periodical and thus proudly displayed in many Italian homes. Denis’s death has led to debates on Italian national radio and TV and long articles about his career in the press.

Born in London, Denis was the son of Wilfrid Mack Smith, a tax inspector in Bristol, and his wife, Altiora (nee Gauntlett). From St Paul’s Cathedral choir school and Haileybury college, Hertfordshire, Denis went to Peterhouse, Cambridge, where he was the organ scholar. In doing so he became the first in his family to go to university.

Denis’s interest in Italy, he later claimed, was first sparked at school, but was also linked to a later encounter with the great liberal historian GM Trevelyan; however, his main reference point at Peterhouse was the historian Herbert Butterfield, a harsh critic of the Whig tradition represented by Trevelyan.

In 1946, Denis spent an extended period in Italy looking at archival material relating to Sicily in 1860. Later he remembered “the hunger and the silence of that period ... I passed entire months without talking to anyone”. The country was still recovering from the trauma of the second world war and people were throwing out books that had enjoyed favour during the fascist period. Denis bought up vast amounts from street stalls and sent them back to the UK, thus forming the basis of a personal library that would aid his writing.

The philosopher Benedetto Croce took Denis under his wing, giving him access to the library of his house in Naples, but only at night. Croce would occasionally join the young scholar, dressed in his night shirt.

In 1949, Denis met one of Garibaldi’s daughters in Rome, and she invited him to visit the family home on the island in Caprera. He later said that it was one of the great regrets of his life that he had not been able to do so.

After serving as a fellow of Peterhouse (1947-62), Denis was elected as a research fellow at All Souls College, Oxford (1962-87). A tall, dashing, charming and elegant man, he was generous with his time and learning. In later years he entertained Italian scholars at his house near Oxford and gave them books from his extensive collection.

His own other publications included a biography of Mussolini (1982), and further, separate studies of Garibaldi and Cavour were joined in 1994 by a biography of Giuseppe Mazzini, the other leading figure of the Risorgimento. Denis also edited The Making of Italy, 1796-1866 (1988), and co-authored A History of Sicily (1986) with Moses Finley and Christopher Duggan.

He had supervised Duggan’s doctorate, and one of the last times he was seen in public came at the memorial service in 2015 for his student. Well into his 90s Denis was still hosting lunch in Oxford and would insist on driving his guests back to the station afterwards at some speed.

Despite, or perhaps because of, his popularity in Italy, many mainstream historians there despised Denis, and wrote scathing reviews of his books. When the Italian historian Renzo De Felice published an explosive interview-book concerning his interpretation of Italian fascism in 1975, Denis replied with his own interview, published as A Monument for Il Duce. A live, primetime debate followed on national TV.

It was a clash of styles and cultures. On one side there was De Felice the powerful academic, barely able to hide his contempt for Mack Smith, and on the other the perfect English gentleman, with a strong foreign accent and subtle point-scoring. Mack Smith started the debate by describing Mussolini as a “criminal”, something that undermined the entire basis of De Felice’s monumental biography. In one of his last academic pieces, Mack Smith picked apart De Felice’s huge biography of Mussolini with meticulous care and forensic attention to detail. He began with this example of perfect understatement: “Renzo De Felice’s unfinished biography of Mussolini is not an easy read.”

Denis’s success set in train a whole host of British scholars working on modern and contemporary Italy. The Association for the Study of Modern Italy, which organises an annual conference and publishes the journal Modern Italy, was set up in 1982, and Denis followed Christopher Seton-Watson as its chair. He was made a fellow of the British Academy (1976), a Grande Ufficiale dell’Ordine di Merito della Repubblica Italiana (1984) and a CBE (1990).

In his historical writing he was never afraid to voice his opinions. As he wrote in the preface to his History of Italy: “He is a coward or a dullard who does not risk some interim judgements on the course of history.”

In 1963, Denis married Catherine Stevenson. She survives him, along with his daughters, Sophie and Jacinta, and his grandchildren, Stephanie, Tosca, Jonah and Theo.

• Denis Mack Smith, historian, born 3 March 1920; died 11 July 2017