How did the unification of Italy end the pope’s temporal power over Rome?

The unification of Italy, known as the Risorgimento, fundamentally challenged the Papal States because they divided the peninsula and were protected by foreign powers, acting as a barrier to national unity.1

Throughout the 19th century, the papacy faced increasing pressure from nationalist and liberal movements. While Pius IX initially showed liberal inclinations, he eventually opposed Italian unification, believing his temporal authority was essential to protecting the church's spiritual independence.12

The end of papal temporal power occurred in stages:
* 1861: Following the defeat of Austria, most papal territories joined the new Kingdom of Italy, leaving only Rome and its surrounding region under papal control, protected by French troops.1
* 1870: During the Franco-Prussian War, French forces withdrew from Rome. On September 20, 1870, Italian troops entered the city, and a subsequent plebiscite confirmed its incorporation into Italy.12

Following this, the Law of Guarantees (1871) attempted to regulate the status of the pope, who remained a self-described "prisoner in the Vatican" until the Lateran Treaty of 1929 established the independent Vatican City.123