How did Italy annex Rome in 1870?

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Unification of Italy — unification of Italy The unification of Italy. The dates are those of annexation, first to Sardinia-Piedmont and after 1861 to the Kingdom of Italy.

The annexation of Rome by the Kingdom of Italy in 1870 was made possible by the outbreak of the Franco-Prussian War, which forced the withdrawal of the French troops that had previously protected the Papal States from Italian forces.12

Following the defeat and abdication of Napoleon III, the pope lost his French military protection. On September 20, 1870, Italian troops breached the city walls at Porta Pia. After a brief, token resistance by the papal army, the Italians entered Rome.1

Following the occupation, a plebiscite was held in October 1870, and Rome subsequently became the capital of Italy. Pius IX refused to recognize the new political reality, withdrawing into the Vatican palace and declaring himself a prisoner, a stance that persisted until the Lateran Treaty of 1929.21