Giuseppe Garibaldi, hero and frontman of the 19th-century Italian national liberation movement, was one of the first political celebrities of the age of press proliferation. During his lifetime, literacy levels across Europe soared, print became progressively cheaper, travel easier, and the public’s hunger for intimacy with the great figures of their age ever more avid.

Garibaldi spent his 30s in South America, fighting in Rio Grande do Sul and Uruguay. Meanwhile, Giuseppe Mazzini, the Risorgimento’s theorist and prolific journalist, was industriously building up Garibaldi’s heroic reputation. Mazzini, himself physically frail, needed a charismatic action man for a figurehead. Garibaldi, as a British diplomat was to explain to Queen Victoria, had “all the qualifications for making him a popular idol” — foremost among them “dash.” By the time he returned in 1848, just in time to catch the wave of revolution sweeping through Europe, he was an international celebrity. Mazzini publicly saluted “the long-awaited Garibaldi,” announcing “his glory is our glory, and is Italian.”

In 1860 — his annus mirabilis — Garibaldi drove the Bourbon armies out of Sicily, after landing on the island with only just over 1,000 untrained volunteers. Reporters swarmed after him. Ivan Turgenev, taking the waters in Baden-Baden, wrote: “What of Garibaldi! One cannot believe it — one’s heart stops beating.” Ever mindful of the importance of public relations, Garibaldi took time out, the night before he attacked Palermo, to sign autographs for some of the hundreds of thrill-seeking volunteers who had joined him.

A few weeks later, he crossed onto the Italian mainland. As he moved up through Calabria, the demoralized Bourbon armies melting away before him, he left his troops behind and galloped ahead, accompanied only by a skeleton staff and a rapidly growing throng of excited tourists. This was warfare as spectacle, conquest by hype. By the time Garibaldi reached the railhead at Salerno, he was two days’ march ahead of his vanguard. He took a train and was met in Naples by the head of the Camorra, who assured him the city was his. The miracle worker Mazzini had created had toppled a monarchy and forged a new nation (there was more to be done, but this was the beginning of modern Italy), and he had done it equipped only with his fame.

It’s a fantastic story. Even now, however rudely it’s debunked, however rigorously it’s deconstructed, it remains one of history’s last great romantic moments. Afterward Garibaldi fell out with the state he had helped found and later described himself as an orange from which his political masters (first Mazzini and later King Victor Emmanuel and his first minister, Count Cavour) had sucked the juice and cast away the peel. But as his real political influence dwindled, his reputation grew ever more inflated. As he hoed his bean rows on a little island off the coast of Sardinia, sightseers crammed into the inn on a neighboring island to gawk at him through their binoculars.

When Garibaldi visited London in 1864, the crowds who turned out to welcome him were so immense it took him six hours to travel three miles through the streets. His host’s servants did good business selling hairs from his comb and little bottles of his used bathwater.

He had become a saint whose relics were to be revered, a handy vehicle onto which people who knew little and cared less about Italian politics could load their fantasies — erotic, political, or nostalgic. When he met Alfred Lord Tennyson, the conversation was limited. Garibaldi, a brilliantly unconventional strategist and an inspirational orator, was not, in any sense of the word, dumb, but he didn’t speak much English, and the poet recorded that he was “not disappointed.” What Tennyson had looked for, and got, from the encounter was not an exchange of ideas, but contact with “the divine stupidity of the hero.” Poor Garibaldi, who had briefly been dictator of half of Italy, had made the degrading transition to stardom from leadership.

In “Garibaldi: Invention of a Hero” (Yale University Press, 496 pages, $35), Lucy Riall painstakingly charts the construction and metamorphosis of his reputation. There’s more detail here than most general readers will want, but Ms. Riall’s material is fascinating and her analysis of it subtle and acute. She explains how the telling of Garibaldi’s story was shaped by contemporary popular fiction: “I like Garibaldi’s expedition,” the French writer Prosper Mérimée wrote, “because I love novels.”

She describes the way Garibaldi, anti-clericalist though he was, appropriated the language and ritual of the Catholic Church and used it to create a new cult of himself. She also demonstrates that the supposedly archaic hero (Garibaldi was frequently compared with the warriors of classical epic or medieval romance) was actually a knowing practitioner of the modern art of public relations, deliberately “staging” his own apparent simplicity.

This is a comprehensive historiography of a 19th-century hero. It is also a timely study of the way wish-fulfillment fantasy, political reality, and popular entertainment interact.

Ms. Hughes-Hallett is the author of “Heroes: A History of Hero-Worship” (Knopf/Perennial).