Law of Guarantees
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After the overthrow of the Papal States in 1870, Italy's Law of Guarantees accorded the Pope certain honors and privileges similar to those enjoyed by the King of Italy, including the right to send and receive ambassadors who would have full diplomatic immunity, just as if he still had temporal power as ruler of a state.
Pope Pius IX and his successors refused to recognize the right of the Italian king to reign over what had formerly been the Papal States, or to be mollified by the Law of Guarantees, and instead considered themselves prisoners of the Vatican, refusing to set foot outside the walls of the Vatican until the Lateran Treaty of 1929 settled the Roman Question.