Why did Pope Pius IX call himself a prisoner in the Vatican?

Pope Pius IX declared himself a "prisoner in the Vatican" following the unification of Italy and the subsequent capture of Rome by Italian troops on September 20, 1870.12

This act was a protest against the Italian government's seizure of the Papal States and the loss of his temporal power, which he believed was essential to protect his spiritual authority. Pius IX adamantly refused to recognize the new political situation or the legitimacy of the Italian state's occupation of Rome. Consequently, he rejected the Law of Guarantees—an 1871 attempt by the Italian government to regulate relations with the papacy—and refused to leave the Vatican palace.1342

This self-imposed confinement, known as the Roman question, persisted for decades as a symbol of the dispute between the church and the Italian state. The situation was not formally resolved until the Lateran Treaty of 1929, which established the independent state of Vatican City.541