This 5 May will mark the bicentenary of Napoleon’s death on St Helena. The occasion will no doubt be marked, as was the bicentenary of the Battle of Waterloo six years ago, by a flood of new books about the emperor, adding yet more to the estimated 200,000 already written. Given this saturation, one wonders if there is anything left to say. This fascinating book proves that there is. It does so by focusing on a crucial yet neglected aspect of Napoleon’s rule: his bitter, decade-long confrontation with Pope Pius VII. This marked an important step both in the emperor’s decline and fall, and in the evolution of the Catholic Church.

It is a dramatic story. Napoleon came to power determined to heal the most gaping wound left by the French Revolution, its schism with the Church, which for almost ten years had fuelled persecutions, peasant risings and civil war across large areas of France. As a partner in this task, he was lucky enough to find Barnaba Chiaramonti, recently elected as Pope Pius VII. The result of this collaboration was a remarkable achievement, the Concordat of 1801, which settled the respective limits of ecclesiastical and civil power in post-revolutionary France and outlived Napoleon by almost a century. The seal was set on the relationship three years later, when Pius travelled to Paris to officiate at Napoleon’s grandiose coronation in Notre-Dame as emperor of the French.

After this pinnacle, relations soured rapidly. With each military victory, Napoleon’s ego expanded. He became increasingly determined to twist the Concordat to subordinate the Church to the secular authorities, insisting on the ultimate right to nominate bishops in his empire, and began to annex outlying parts of the papal territories in central Italy. In February 1808, he sent his troops to occupy the Papal States. As Ambrogio Caiani makes clear, using significant, previously unexploited material in the Vatican Archives, Pius had seen this trial of strength coming and spent several months preparing for it. He ordered his bishops and priests not to cooperate with the invaders, embarked on a policy of passive resistance and finally excommunicated the emperor. In retaliation, Napoleon took the extraordinary step of ordering the pope’s arrest and removal from Rome. On the night of 5 July 1809, this was carried out in a farcical operation involving collapsing ladders and soldiers blundering about in the dark, which Caiani recounts with verve. The pope was first placed under house arrest in Savona on the Ligurian coast, then in June 1812 transferred to the palace of Fontainebleau, south of Paris, where, courageously, he continued to resist Napoleon’s browbeating. He was only released when the French empire fell in the spring of 1814.

Caiani relates this dramatic story in telling detail but never loses sight of the broader picture, and uses his archival discoveries to excellent effect. The result is both an exciting narrative and a fine work of scholarship, shedding new light on Napoleonic history and that of the modern Catholic Church. In particular, he pinpoints Napoleon’s worst aspects: his obsession with domination, his complete inability to negotiate with equals rather than dictate to subordinates and his terrible temper. In January 1813, a rumour spread that, during a stormy interview at Fontainebleau, he had actually struck the pope. Rather generously, Pius himself later set the record straight, saying that Napoleon ‘never undertook such an excess against my person. However, one day in the heat of the moment … he grabbed me by one of the buttons of my cassock and shook me so strongly, by pulling it, that my entire body stirred. It is probably to this incident that most are referring.’ One might think that Pius was making a rather fine distinction. For historians, as for his more discerning contemporaries, Napoleon does not improve on closer acquaintance.

However, the book’s real originality lies not in exposing Napoleon’s flaws but in bringing his neglected nemesis, Pius VII, out of the shadows. Sixty-seven at the time of his kidnapping, Pius was physically frail, but his unassuming exterior hid a core of steel. He had initially been open to reform of the Church, but Napoleon’s campaign of intimidation hardened his determination not to surrender any of his spiritual and temporal powers. The ordeals of his imprisonment make for painful reading. For long periods, this elderly man was kept in isolation, without even pen and paper. At other times, his captors constantly pressured him to submit to their master by feeding him exaggerated stories about the Church being in crisis with its head in captivity. In Savona, spyholes were drilled into the walls of his apartments, one directly above his bed, and on one occasion his bedroom was ransacked and his mattress torn to shreds in search of illicit correspondence. Small wonder that these measures took their toll: Pius had panic attacks and developed inflammation of the bladder and stomach; at times he lapsed into catatonia. As Caiani makes clear for the first time, during his transfer from Savona to Fontainebleau the pope came very close to death.

Despite these tribulations, Pius steadfastly refused to concede his power to appoint bishops to Napoleon. One of the most important contributions of this book to scholarship on Napoleon is its examination of the wider implications of Pius’s stand. It set the Catholic Church across Europe in opposition to the imperial regime and ignited the first major civil resistance to its rule. In this respect, it was the initial stage in the disintegration of the Napoleonic empire, and it set a powerful precedent. In July 1811 a national council of the Church summoned by Napoleon to confirm his usurpation of Pius’s powers refused to do so; two years later France’s legislative body followed suit, denounced the emperor’s warmongering and demanded he negotiate peace with his enemies. Ultimately, Napoleon lost his throne thanks less to military defeat (his opponents were prepared to negotiate with him almost until the end) than to his infallible ability to alienate his natural allies.

In the short term, the pope won his epic battle with the emperor. In May 1814, Pius returned in triumph to Rome and resumed the papal throne. In contrast, Napoleon was exiled first to Elba and then to St Helena, where he relieved the tedium of the last six years of his life by dictating mendacious memoirs to his entourage. Yet readers seeking a happy ending to this tale will be disappointed. Pius, who had begun his reign as an open-minded reformer, returned to Rome an ailing and suspicious reactionary, permanently marked by the rigours of his imprisonment at Savona and Fontainebleau. He re-established the Inquisition, locked Rome’s Jews back into their ghetto and guarded his own prerogatives ever more jealously. By the time he died in 1823, as Caiani tactfully puts it, ‘half a decade of imprisonment and confinement had sapped his tolerance and his patience’. From the early Middles Ages to the Napoleonic era, Europe was regularly convulsed by struggles between pope and emperor, Church and State. In this fascinating episode of that ancient conflict, both parties emerged the losers.