The Papal States reached their largest territorial extent in 1598, when Pope Clement VIII took advantage of the exhaustion by all major European powers with regards to Italy to occupy the Duchy of Ferrara following the death of the childless Duke Alfonso II. Alfonso had attempted to preserve the independence of his duchy by nominating his cousin, Cesare, as his heir. In spite of his designation as heir, Cesare was unable to oppose the Papal occupation, although he did manage to set up a rump state centered around the city Modena.

The Papal territory would remain unchanged until 1791, when the French Revolutionary Government annexed the Papal fief in Avignon, however the contiguous Papal States would remain unchanged until their annexation by the nascent Kingdom of Italy in two phases, in 1859 and 1870.