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Why did the Papal States end in 1870?

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Vatican City: St. Peter's Basilica — papacy St. Peter's Basilica, Vatican City.

The Papal States came to an end in 1870 as a direct result of the Risorgimento, the 19th-century movement for Italian unification. Throughout this period, the existence of the Papal States was viewed as a significant obstacle to national unity because they geographically divided the Italian peninsula and relied on foreign military protection to maintain papal sovereignty.12

By 1861, most former papal territories had already been annexed by the new Kingdom of Italy. Rome and its surrounding region remained under papal control only because they were defended by French troops. However, during the Franco-Prussian War in 1870, these French forces were withdrawn, leaving the city vulnerable. Italian troops subsequently entered Rome on September 20, 1870, and following a plebiscite, the city was incorporated into the Italian nation as its capital.12

This event triggered the Roman question, a long-standing dispute over the loss of papal temporal power that was not formally resolved until the Lateran Treaty established Vatican City in 1929.13