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What happened when Italian troops entered Rome on September 20, 1870?

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Side-by-side maps of central Italy showing the changing boundaries of the Papal States. The left map highlights the Papal States after the Treaty of Vienna in 1815 in green, showing their greatest extent. The right map shows the Papal States' reduced territory just before absorption into Italy, with orange marking areas annexed in 1860 and yellow showing the final area annexed in 1870, centered ar — Papal States The Papal States in 1815 (left) and at their annexation by Italy in 1870.

When Italian troops entered Rome on September 20, 1870, they did so following the withdrawal of French military protection, which had been necessitated by the Franco-Prussian War. After encountering token armed resistance from the papal army, the Italian forces breached the city walls at Porta Pia and occupied the city.12

This event marked the final collapse of the Papal States as a temporal power. In October 1870, a plebiscite was held in which an overwhelming majority of voters supported the incorporation of Rome into the Kingdom of Italy, and the city subsequently became the nation's capital.13

Pope Pius IX refused to recognize the legitimacy of the Italian occupation or the loss of his temporal authority. He withdrew into the Vatican palace, declaring himself a "prisoner," a stance maintained by his successors until the Lateran Treaty of 1929 established the independent state of Vatican City.132