Complex sentence: They 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 by making Vatican City an independent state and paying compensation to the Vatican for the loss of the Papal States.
Split into simpler sentences: They instead considered themselves prisoners of the Vatican, refusing to set foot outside the walls of the Vatican until the Lateran Treaty of 1929. The Treaty settled the Roman Question by making Vatican City an independent state and paying compensation to the Vatican for the loss of the Papal States.