The Law of Guarantees
It is usually taken if any other nation than Papal States forms Italy.
Other than the obvious advantages to prestige and infamy-reduction, Rome is a more defensable position than most of the alternative capitals. So it is advised to take this decision as soon as possible if Italy does not have below 2 infamy.
In 1870 The law of Guarantees was a law passed by the newly formed Kingdom of Italy -- aimed at solving the 'Roman Question' -- that accorded the pope priviliges similar to those of the king himself, including the right to send and receive ambassadors. Pope Pius IX refused to accept the law, which was seen both by him and by his successors as an infringement of the independence of the Holy See. While the issue was not conclusively solved until the Lateran Treaty of 1929, the Law of Guarantees nevertheless meant that the young kingdom's capital would from then on be Rome.