When Liane de Pougy, one of the most celebrated Parisian courtesans, visited Florence, a famous admirer sent a carriage filled with roses to collect her. As she descended the steps, his servants threw more roses at her. "There before me was a frightful gnome with red-rimmed eyes and no eyelashes, no hair, greenish teeth, bad breath, the manners of a mountebank and the reputation, nevertheless, for being a ladies' man."

This was none other than Gabriele D'Annunzio, the poet and lothario who seduced Italy to wartime slaughter with his rhetoric, scandalised Europe with his writing and set up his own city state in a forerunner of fascism. In this exhaustive biography, Lucy Hughes-Hallett attempts to peel away the many layers of an astonishing Italian egotist who still divides opinion over his politics, poetry and prose.

He was, without doubt, a revolting man, whose rampant vanity and sexual desires knew no bounds. Although he bedded scores of Europe's most beautiful women, his treatment of them was contemptuous; indeed, there are suggestions from his writing he liked the idea of raping working-class women. His housekeeper was expected to have sex with him three times a day.

Then there was his bloodlust as he sought Italian participation in the first world war, with fiery nationalist speeches and sub-Nietzschean fantasies, arguing a race only won respect by spilling the blood of its young. Even his biographer admits she is repelled by him. Once at war, he orders soldiers to shoot some captive countrymen whom he called "sinners against the fatherland". Little wonder he captivated Mussolini.

Yet he was brave in battle, a passionate protector of his men, a pioneering aviator. Above all, he was a prodigious writer whose collected works ran to 48 volumes. Puccini wanted to work with him, Proust admired him and Joyce said he was one of the three most talented writers of the 19th century, alongside Kipling and Tolstoy. His flowery and explicit writing had flair, even if he was not, as he claimed, the greatest Italian writer since Dante. But then, even his children had to call him maestro.

It all makes a splendid subject for a biography, although since he wrote constantly in his notebooks, there is a surfeit of material and at times this biography sags slightly as it tries to make sense of such a well-recorded life. There were rumours he removed his ribs to perform fellatio on himself; he claimed to have eaten the meat of children; there is drug use towards the end of his life as his health deteriorates. Some stories were false, of course, made up by D'Annunzio or reporters soaking up his life for their papers.

Here lies the key to this horrifically fascinating subject. For he was not just the prototype fascist who paved the way for Mussolini, but a pioneer of modern celebrity culture. He understood the fantastic soft power of fame. So while still a teenager, he published a volume of poetry, then informed a newspaper editor the young writer was dead, ensuring national publicity. When the Mona Lisa was stolen, he claimed it was brought to his house. After he sat on a plane at an air show, mechanics auctioned the seat to fans.

His greatest work of art was the construct of Gabriele D'Annunzio. "The world must be convinced that I am capable of anything," he wrote, and in his life, he lived up to this ideal. He was undeniably brilliant – at the age of 16, he wrote to his parents in six languages. The tragedy was that he put this genius to such nefarious ends, fanning the flames of war, nationalism and blood-stained division that culminated in such tragedy for his country and continent.

Hughes-Hallett dances her way through this extraordinary life in a style that is playful, punchy and generally pleasing. She eschews chronology in places for a chopped-up style of vignettes that works surprisingly well as she seeks to separate the man from his myths. Mostly, she allows the poet to hang himself. And she shows the links between him and Mussolini are more blurred then suspected, with D'Annunzio constantly wary of the emerging fascist leader.

Indeed, he seems bored by politics, with few fixed convictions beyond his own importance and a crude sense of Italian greatness, while Mussolini watches and learns from the master of self-promotion. The best bit of the book is the description of the anarchic events at Fiume in 1919, when black-shirted nationalists seized the Adriatic port for Italy. For more than a year, D'Annunzio is duce of it as it descends into darkness and racist divisions, a portent for scenes soon to engulf Europe. Meanwhile, he changes the flowers round his bed three times a day.

He ends his life promoted to general and living in Lake Garda, turning his home into a temple to himself. Mussolini, realising the potency of the poet's appeal in Italy, smothers him with luxury, sending him ever more outrageous gifts for his garden, culminating in a plane and the prow of a battleship. After his death in 1938, his girlfriend turns out to have been a Nazi agent; there are rumours she killed him with a drug overdose. In death, as in life, the amazing story of D'Annunzio was painted in primary colours, but with the darkest of shadows.

Lucy Hughes-Hallett discusses The Pike at Lutyens & Rubinstein book shop, 21 Kensington Park Road, London W11 on Wednesday 6 February at 7pm (£5, including a glass of wine)