- French Emperor Napoleon III would withdraw all French troops from Rome within two years.
- King Victor Emmanuel II of Italy guaranteed the frontiers of the Papal States, which at the time consisted of Rome and Latium.
- The Italian government would move from Turin to Florence, to indicate that the government would not attempt to take its seat in Rome. (Rome had been declared Capital of Italy on 27 March 1861, when the first Italian Parliament had met in Turin.)
This treaty was opposed by the Pope, the French Catholics, and by Italian patriots. When the government’s move to Florence was announced, widespread anti-government riots broke out, and hundreds of Italian civilians were killed. The last French troops left Rome in December 1866. Napoleon III hoped that the Italian government and Pope Pius IX would negotiate a compromise that would allow the government to move from Florence to Rome.
Because the intransigent Pius IX rejected all proposals, Italian patriots, under the leadership of Giuseppe Garibaldi, organized an invasion of Latium and Rome in October 1867. The patriots were defeated at Mentana by 2,000 French troops, sent by Napoleon III. A French garrison was kept in Rome to prop up the rule of Pius IX.
In August 1870, following the outbreak of the Franco-Prussian war, the French garrison was recalled from Rome. Widespread public demonstrations demanded that the Italian government take Rome. The Italian government took no direct action until the defeat of Napoleon III at the Battle of Sedan, after which the Italian government was no longer bound by the September Convention. Victor Emmanuel sent Count Gustavo Ponza di San Martino to Pius IX with a personal letter offering a face-saving proposal that would have allowed the peaceful entry of the Italian Army into Rome, under the guise of offering protection to the Pope.
- The Pope’s reception of San Martino (10 September 1870) was unfriendly. Pius IX allowed violent outbursts to escape him. Throwing the King’s letter upon the table he exclaimed, "Fine loyalty! You are all a set of vipers, of whited sepulchres, and wanting in faith." He was perhaps alluding to other letters received from the King. After growing calmer, he exclaimed: "I am no prophet, nor son of a prophet, but I tell you, you will never enter Rome!" San Martino was so mortified that he left the next day.
The Italian Army, commanded by General Raffaele Cadorna, crossed the Papal frontier on 11 September and advanced slowly toward Rome, hoping that a peaceful entry could be negotiated. The Italian Army reached the Aurelian Walls on 19 September and placed Rome under a state of siege. Pius IX refused to surrender and the Papal Zouaves kept resisting. On September 20, after a cannonade of three hours had breached the Aurelian Walls at Porta Pia, the Bersaglieri entered Rome and marched down Via Pia, which was subsequently renamed Via XX Settembre. 49 Italian soldiers and 19 Papal Zouaves died. Rome and Latium were annexed to the Kingdom of Italy after a plebiscite. The Pope declared himself a "prisoner in the Vatican". The following year, the Italian government moved from Florence to Rome.
Historian Raffaele De Cesare made the following observations:
- The Roman question was the stone tied to Napoleon’s feet--that dragged him into the abyss. He never forgot, even in August 1870, a month before Sedan, that he was a sovereign of a Catholic country, that he had been made Emperor, and was supported by the votes of the Conservatives and the influence of the clergy; and that it was his supreme duty not to abandon the Pontiff.
- For twenty years Napoleon III had been the true sovereign of Rome, where he had many friends and relations [...] . Without him the temporal power would never have been reconstituted, nor, being reconstituted, would have endured.
- De Cesare, Raffaele (1909). The Last Days of Papal Rome. London: Archibald Constable & Co.