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What happened when Italian troops entered Rome in September 1870?

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Photograph of Pope Pius IX — Pius IX Pius IX is the longest-reigning pope in the history of the Roman Catholic Church. His pontificate lasted from 1846 to 1878.

When Italian troops entered Rome on September 20, 1870, they effectively ended the Papal States and the pope's temporal power. Following the withdrawal of French protection during the Franco-Prussian War, Italian forces breached the city walls at Porta Pia after encountering only token resistance from the papal army.12

In October 1870, a plebiscite was held in which an overwhelming majority of voters supported the incorporation of Rome into the Kingdom of Italy, making it the nation's capital. Pius IX adamantly refused to recognize the new political reality or the loss of his temporal authority. He withdrew into the Vatican palace, declaring himself a "prisoner" and refusing any formal intercourse with the Italian government. This state of affairs, known as the Roman question, remained unresolved until the Lateran Treaty of 1929 established the independent state of Vatican City.1342