The Papal States are conventionally dated from 756, when the Frankish king Pepin the Short, having defeated the Lombards on the pope's behalf, granted the reconquered territory to the papacy rather than restoring it to Byzantine rule, an act known as the Donation of Pepin. That grant made the pope a temporal ruler over central Italian territory in addition to his religious authority, a combination that persisted, with fluctuating borders, for well over a thousand years.
The arrangement created a durable structural tension that recurs throughout the period's history: the pope was simultaneously a bishop answerable to religious duties and a prince running armies, courts, taxation and diplomacy like any other Italian ruler, and this double role was argued over inside the church as much as by its rivals outside it. The territories' size and borders shifted considerably across the centuries through war, inheritance disputes and periodic loss and recovery of specific cities, but the core papal territory around Rome remained continuously under papal temporal rule until the nineteenth century.
The end came in 1870, when the Kingdom of Italy annexed Rome and the remaining papal territory as the final act of Italian unification, over the pope's explicit and unresolved objection.