Lateran Treaty
Negotiations for the settlement of the Roman Question began in 1926 between the government of Italy and the Holy See, and culminated in the agreements of the Lateran Pacts, signed—the Treaty says—for King Victor Emmanuel III of Italy by Benito Mussolini, Prime Minister and Head of Government, and for Pope Pius XI by Pietro Gasparri, Cardinal Secretary of State, on 11 February 1929. It was ratified on 7 June 1929. The agreements were signed in the Lateran Palace, hence the name by which they are known.
Pope Pius IX
Pius IX was the last pope who was also a secular ruler as monarch of the Papal States, ruling over some 3 million people. In 1870 the Papal States were seized by force of arms by the newly founded Kingdom of Italy. The matter was only resolved in international law by the Lateran Treaty (also known as the "Lateran Pacts" or "Lateran Accords"), agreed in 1929 between the Kingdom of Italy and the Holy See, the latter receiving financial compensation for the loss of the Papal States, in substitution of which Italy recognized the Vatican City State as an independent territorial state which is the expression of a sovereign entity in International law known as the Holy See. The latter, as before, maintains diplomatic relations with many other states.