For most of the modern period the pope was also a temporal ruler, governing the Papal States across central Italy. Italian unification took those territories, and in 1870 Italian troops entered Rome itself. The papacy refused to accept the loss, declined to leave the Vatican palace, and for the next fifty nine years successive popes described themselves as prisoners and stayed inside the walls. David Kertzer's account of the period examines how that refusal was maintained and what it cost both sides, including a papal instruction discouraging Italian Catholics from voting in the new state's elections.
The dispute was settled in 1929 by the Lateran treaty, which recognised Italian sovereignty over the former papal territories, created the Vatican City State, and provided compensation. That settlement is why the modern Holy See has a territory at all, and why the territory is deliberately tiny: the point was never to govern a population but to guarantee that no other government could reach the See. The period is worth recording in a religious atlas because it shows a religious institution operating for two generations with international standing, a diplomatic corps and worldwide authority while occupying about fifty hectares by refusal rather than by right.