When Clara Petacci, known as Claretta, first met Mussolini in April 1932, she was a gushing, busty young Fascist of twenty, with dark hair, a prominent nose and good legs; he was forty-nine, shaven-headed, with a jutting chin, a fleshy mouth and burning eyes, and he had been dictator of Italy for the past eight years. The encounter took place at the seaside resort of Ostia, where both had gone to enjoy a fine spring day. Emerging from her chauffeur-driven family car, Claretta bounded over to pay homage to the Duce. Mussolini was gracious. Although never as pretty, as intelligent or as interesting as many of his other lovers, Claretta became his most enduring, and it was she who shared his gruesome end.

Claretta is far less well known than Hitler’s final lover, Eva Braun (who, like her, was born in 1912), particularly outside Italy, where Mussolini’s home town of Predappio remains a shrine for neo-fascists and where hundreds of memoirs by his family and his mistresses have been published. What makes R J B Bosworth’s new book so captivating is not just the picture he paints of their affair, based on diaries, letters and police reports, but also the light he throws on the corrupt, greedy, scheming world of the Fascist leadership. Claretta herself was certainly venal, but there is something touching about her obsessive need for the vain and despotic Mussolini.

Claretta was a child of Fascism. A teenager when Mussolini took power, she grew up to venerate the Duce, whose features were to be seen on every classroom wall, on coins, on posters and on sculptures. Like other Fascist children, she wore the party uniform, extended her arm in the official salute, wrote swooning letters to the Duce, sang ‘Giovinezza’ and learned to be a donna autentica, a model of compliance and subservience, with wide, child-bearing hips, good bones and a placid disposition (as opposed to a donna crisi, a thin, ambitious feminist). Her father was a fashionable doctor; her mother was a ruthless and avid seeker of the preferment and privileges that closeness to Mussolini conferred.

Claretta became Mussolini’s mistress some time in the spring or early summer of 1936. Bosworth describes, at enjoyable length, Mussolini’s many other relationships, some of which endured through Claretta’s tenure. His lovers included Leda Rafanelli, an anarchist with bookish tastes and strong views about gender, Margherita Sarfatti, a rich Venetian journalist, and Ida Dalser, a beautician, who would later shout ‘coward, pig, assassin, traitor’ under his window. Eight of these women bore him at least nine children; Rachele, his wife, gave him another five. Mussolini’s sexual prowess was the stuff of legend. As one commentator observed, his government was less ‘tyrannical’ than ‘priapic’.

Although Bosworth writes about public life in Fascist Italy – the crushing uniformity, the bloody Ethiopian wars, the disastrous intervention in Spain – the Mussolini who emerges from Claretta’s diaries, written from the day they first met, is the private man who, once the famously brief and brutal acts of copulation were done, liked to listen to Beethoven’s Seventh Symphony and mull over the travails of family life. Claretta recorded their lovemaking sessions with a si, which she underlined, but she could also be prim, using only the first letter when writing such words as ‘whore’ and ‘bum’. Through her eyes, we see Mussolini sneer at the British (whom he regarded as drunken, brainless pigs), mock the French for being syphilitic and cowardly, and express awe at his German masters (though he was also capable of deriding Hitler, famously saying, as he returned from a meeting with him in Venice, that he looked like a ‘plumber in a mackintosh’). For all his bombastic anti-Semitism, it is worth remembering that no Jews were deported from Italy before the German occupation and that during the time Italy controlled a part of southeast France, the area became a haven for Jews fleeing Vichy and the Germans in the north.

After Mussolini was ousted from power by the Grand Council in July 1943, the Petaccis, along with the families of other leading Fascists, were forced to flee, leaving their sumptuous houses to be ransacked. Claretta herself was captured and put into prison in Novara, where she complained bitterly about the fleas and cockroaches. In response to the armistice Italy negotiated with the Allies, the Germans occupied much of the country. Mussolini was restored as a puppet dictator with his headquarters in Salò on Lake Garda; Claretta was released and settled in a villa not far from him. Although the two met infrequently over the twenty months that remained of their lives, their squabbling, jealous, intense correspondence continued, and she was with him in April 1945 when a group of partisans intercepted the convoy of cars carrying the Duce to safety in Switzerland or Germany. The image of their two bodies, Claretta with her skirts tied around her knees in an effort to preserve a shred of modesty, swinging upside down above the petrol pumps of a half-built service station in Milan’s Piazzale Loreto, is one of the defining photographs of Italy’s twenty-year dictatorship.

Claretta’s diaries and letters have a history of their own. Buried by friends for safekeeping, they were placed in Italy’s state archives in 1950, at which point the Petaccis sued for their return, won their case, but then lost it on appeal. They have never been translated into English.

Bosworth, the author of some twenty books on the Mussolini years, is one of the finest historians of modern Italy, and his deep knowledge and understanding, as well as his formidable research, inform every page of this enjoyable biography. It is, he says, a love story, one suffused with jealousy, passion, betrayal and forgiveness. But it is also a vivid and rare picture of a time when Italy was led astray by a man who yearned to return his country to the glories of the Roman Empire, and who found time, while governing a nation, to enjoy countless seductions and complicated, overlapping relationships with women.