What happened at the breach of Porta Pia in 1870?

On September 20, 1870, Italian troops breached the city walls of Rome at Porta Pia, effectively ending the Papal States and the temporal power of the pope.12

This event occurred after the Franco-Prussian War forced the withdrawal of the French garrison that had previously protected the city. Following a token resistance by the papal army, the Italian forces entered the city, and a subsequent plebiscite in October 1870 resulted in an overwhelming vote for Rome’s incorporation into the Kingdom of Italy.123

In response to the occupation, Pius IX refused to recognize the new political reality. 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.123