The Legion of Antibes was recruited under direction of Pope Pius IX's secretary of state, Giacomo Antonelli, following the September Convention of 1864, to replace French troops garrisoned in Rome, during the closing phase of Italian unification, the Risorgimento. The September Convention permitted the Pope to keep an army, but it did not give Napoleon III the right to continue to maintain forces in Rome, supporting papal temporal power in the rump Papal States. Nevertheless, at the battle of Mentana, fought 3 November 1867 between French-Papal troops and the Italian volunteers led by Giuseppe Garibaldi, it became public knowledge that the "Legion" was composed of French Imperial recruits, Antibes lying on the Mediterranean coast close to the border with Piedmont: their "services to the Pope were rendered as services to the French Empire," the former Prime Minister of Italy, Francesco Crispi recalled in 1891. "It is singular that on the dead bodies were found livrets French regiments, to which every one of these men belonged, with the number of his matriculation, and with formula of the oath of fidelity to the Emperor."
No formal denunciation of French intervention so contrary to the spirit of the September convention was lodged, however. A frank remark of the French minister Jules Favre with the Italian ambassador Costantino, count Nigra, 6 September 1874, allowed the obvious: "The convention of September 15th is very dead".
Cardinal Antonelli continued to call for the Catholic powers of Europe to come to the aid of the Pope, but there was no response.