On 7 August 1915, Gabriele d'Annunzio and Giuseppe Miraglia took off from Venice in a plane bound for Trieste, then still part of the Austro-Hungarian empire, with a cargo of bombs and propaganda. Miraglia, the pilot, was a 32-year-old army officer; d'Annunzio, in the passenger seat, 20 years older than his companion, was famous throughout Italy, Europe and beyond as a writer, libertine and rabid nationalist. He had a notebook with him, in which he jotted down his impressions of Venice from the air ("the twisting canals are green as malachite") and passed messages to Miraglia: "Would you like some coffee?"

Reaching Trieste, they came under fire. Miraglia brought the plane in low over the marina. D'Annunzio dropped bombs on the Austrian submarines and pamphlets (he'd written them himself) into the piazzas. Turning back, they realised one of the bombs had got stuck. "See if you can push it so it falls out," Miraglia wrote in d'Annunzio's notebook. "But don't twist it." He must have managed it somehow, because they landed safely back in Venice. D'Annunzio "had embarked on his new life as national hero", Lucy Hughes-Hallett writes in her enthralling new biography of the "poet, seducer and preacher of war".

He was born in Abruzzo in 1863. His father, a landowner and wine merchant, was the mayor of Pescara, on the Adriatic coast. At the age of 11 d'Annunzio was sent to boarding school in Prato, 400 kilometres away across the Apennines. His first book of poems, Primo Vere, was published when he was 16, at his father's expense. It was well reviewed in the influential cultural weekly Fanfulla della Domenica. Just before an expanded second edition came out the following year, a newspaper editor in Florence received an anonymous postcard from Pescara saying that d'Annunzio had fallen off his horse and died. The news, reported in papers all over Italy, wasn't even remotely true: d'Annunzio  already displaying not only his precocious talents as a writer but also his gift for self-promotion  had sent the postcard himself.

After that, there was no stopping him. Soon enough he was in Rome, writing prolifically  poetry, journalism, short stories; spending prodigiously  however little or however much he was earning, he always lived way beyond his means, with a fatal weakness for lavish interior décor; and sexual promiscuity. The pattern for his life was set. For the next 20 years he would punctuate his otherwise non-stop thrill-seeking  horse-riding, fox-hunting, motoring, flying, and endless, endless sex  with periods of withdrawal for writing. He wrote the first of his seven novels, Pleasure, in five months in 1888, shut away in a friend's house in Abruzzo. The hero, Andrea Sperelli, is a worldweary young libertine about Rome. Henry James admired its "splendid visual sense" and "ample and exquisite style" (one of the reasons it seems rather dated now). It also sold fantastically well, transforming d'Annunzio into an international celebrity.

A problem for a biographer is that d'Annunzio's personal appeal seems to have lain largely in his voice and in his sexual prowess, which makes it difficult to convey on the page. His insatiable sexual appetite left a trail of ruined women, variously disowned by their fathers, abandoned by their husbands and committed to insane asylums. His superhuman selfishness, his foaming nationalist rhetoric about soaking the earth with blood: they're clear enough. But we simply have to take everyone's word for it that he was irresistibly attractive (well, not quite everyone's: Hemingway thought he was a "jerk", and the courtesan Liane de Pougy called him "a frightful gnome with red-rimmed eyes and no eyelashes, no hair, greenish teeth, bad breath and the manners of a mountebank"). One of the best descriptions of d'Annunzio's mesmerising voice comes from the teenage daughter of the composer Pietro Mascagni: "When Signor d'Annunzio speaks, it always seems as though he is telling one a secret. Even if he is only saying good morning." She met him in Paris, where he'd fled in 1910, no longer able to evade his creditors in Italy.

He returned home in triumph in May 1915, invited to speak at the unveiling of a monument to Garibaldi in Quarto, near Genoa. He turned his magical voice on the vast crowds that gathered to greet him  100,000 of them, according to the Corriere della Sera, when he reached Rome on 12 May  calling for Italy to enter the war and complete the unification of the country by annexing great swathes of the Austro-Hungarian Empire. On 23 May, Italy declared war on Austria-Hungary  though, as it happened, the decision had been irrevocably made while d'Annunzio was still in France.

The mass slaughter and three-year stalemate in the Dolomites did nothing to dim d'Annunzio's enthusiasm for war. He lost many friends, including Miraglia, and was permanently blinded in one eye when a plane he was in was shot down. But after the armistice he wrote: "I smell the stench of peace." And the war for d'Annunzio wasn't over: in September 1919 he led an army of irregulars and mutineers into the disputed city of Fiume (now Rijeka in Croatia) and set himself up as dictator. For 15 months he reigned as Duce, until the Italian navy bombed him out.

In February 1921 he moved into the house above Lake Garda where he would live in semi-seclusion until his death in 1938, dedicating himself to the delights of cocaine, coitus and interior design. His retirement was largely funded by the fascist government, who were keen to keep him quiet and out of the way. Mussolini wanted to promote d'Annunzio as the John the Baptist of fascism, which was easier to do if the man himself, who didn't see things that way, wasn't around to argue.

"Though d'Annunzio was not a fascist," Hughes-Hallett observes, "fascism was d'Annunzian." In 1892 he had written that "men will be divided into two races. To the superior ones, who have raised themselves by the pure energy of their will, everything will be permitted, to the inferior ones nothing, or very little." Admiring some of the work, and insisting on looking at d'Annunzio in context, Hughes-Hallett does her best to withhold judgment on the life: she is, she says, "a woman writing about a self-styled 'poet of virility' and a pacifist writing about a warmonger, but disapproval is not an interesting response". Having said that, she calls him "appalling" on several occasions. As indeed in many ways he was. Disapproval may not always be interesting, but sometimes it's unavoidable.

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)