The Capture of Rome (Presa di Roma) was the final event of the Italian unification known as Risorgimento.
In 1870, Rome was still part of the Papal States (Stato Pontificio), territories under the rule of the Catholic Church from 752 to 1870 that covered most of the regions of Emilia Romagna, Marche, Umbria and Latium.
After the Italian Independence Wars and Garibaldi’s Expedition of 1860 that unified Italy, only the Latium region was left out, until the Italian army, led by general Raffaele Cadorna, entered Rome on September 20th through Porta Pia, following a cannonade of three hours.
The enemy, the papal force, was commanded by General Hermann Kanzler, and was composed of the Swiss Guards and a few "zouaves"—volunteers from France, Austria, the Netherlands, Spain, and other countries—for a total of 13,157 men against some 50,000 Italians. General Cadorna had his men distribute flyers with a declaration:
"Romans! The excellence of our right and the valor of our arms have in a few hours brought me among you to restore to you liberty. Now your destinies, those of the nation itself, lie in your own hands. Strong by your sufferings, Italy will at least have the glory of solving that great problem which has been so terrible a burden to modern society. Thanks Romans, also, in the name of the army, for the heartfelt reception you have given us! Continue to preserve as you did to this day public order, because without it there is no liberty possible. Romans! the morning of the 20th of September 1870 makes a memorable epoch in history. Rome is again restored, to be now and forever the great capital of a great nation. Long live the King! Long live Italy! R. Cadorna."
Rome and its region, Latium, were officially annexed to the Kingdom of Italy after a plebiscite held on October 2nd: 133,681 men voted for annexion, 1,507 opposed (in Rome itself, there were 40,785 "Yes" and 57 "No").
For many years afterwards, the Catholic Church rejected the Roman annexation until the Concordat of 1929, where the Church renounced its claims over most of the city of Rome in return for Italy's recognition of the Vatican State.
New-York Tribune, September 28, 1870
A political map of Italy in early 1796, before the Napoleonic wars, created by MapMaster.