On 19 September 1870, the armies of the Kingdom of Italy laid siege to Rome in their desire to incorporate the city into a unified Italian state. On the 20th of September, the Italian armies will capture the city ending more than one thousand years of temporal rule by the papacy. Popes Pius IX through Pius XI will refuse to recognize the loss of Rome and call themselves the “Prisoner of the Vatican”.
With the defeat of Napoleon III at the Battle of Sedan, the French Government was no longer willing or able to prevent the desires of many Roman citizens as well as the Italian Government itself that Rome become part of the Kingdom of Italy. An attempt by King Victor Emmanuel III to get Pope Pius IX to acquiesce to Italian troops marching into Rome under the guise of protecting the Pope was rejected. The Italian government hoped that a peaceful surrender could be negotiated and as the Italian army approached Rome, Pius IX recognized that he could not defend the city but would put up a token resistance.
On 19th of September, the Italian army reached the Aurelian Walls of Rome. On 20th, Italian troops will enter Rome and after a brief struggle during which 68 soldiers were killed, Italy captured the city. On the 21st, the Italian forces will capture the Leonine City, including the Vatican. The Italian government had attempted to offer a deal to Pius IX to offer the Leonine City in return for his recognition of the loss of Rome but Pius IX refused.
For the next 59 years, the popes refused to give any sign that they recognized the authority of the Italian government. They would not appear in Saint Peter’s Square or leave the Vatican. Only when Pope Pius XII agreed to the Lateran Treaty in 1929 establishing an independent Vatican City did this situation change.