The September Convention was a treaty, signed on 15 September 1864, between the Kingdom of Italy and the French Empire, under which: Additionally, in a protocol at first kept secret, the Italian government pledged to move its capital from Turin to another city (later selected by a commission to be Florence) within six months, to prove its good faith in giving up all claims on Rome.
[2] According to the terms of the treaty, Napoleon III would withdraw all French troops from Rome within two years; and King Victor Emmanuel II of Italy would guarantee the territorial integrity of the Papal States.
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.
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.