The problem of reconciling the inalienable right of the Holy See to temporal sovereignty with the natural desire of the Italians for a united nation with Rome as the Capital, since the necessary conditions of this sovereignty would be immunity from subjection to any civil ruler, and civil jurisdiction over a state or sovereign territory.
The Question arose when the Italian troops occupied Rome on 20 September 1870.
In protest against the usurpation of papal territory, the popes remained voluntary prisoners in the Vatican for 59 years, during the greater part of which sectarian prejudice and hatred kept Italy from coming to terms.
On 11 February 1929, the Question was settled by the Treaty of the Lateran, in which Italy recognized both the sovereignty of the Holy See as a national entity, and the City of the Vatican as territory independent of Italy, abolished the Law of Guarantees, and settled all financial relations by payment of 750,000,000 lire cash, and 1,000,000,000 in Italian state console at five per cent; the Holy See in turn renounced its rightful legal claim to the City of Rome, and the old Papal States.
New Catholic Dictionary