The expression the Papal States - always in the plural and with the definite article - refers to the territory in central Italy over which the popes ruled from the eighth century until Italian unification in 1870. The extent of this territory, as well as the degree to which the popes were able to exercise effective control over it, varied considerably, but at its most extensive, in the eighteenth century, it included the present-day regions of Lazio, Umbria, the Marche, and the southern half of Emilia-Romagna. The Papal States are sometimes called the States of the Church.
Do not confuse the Papal States (plural) wirh the Papal State (singular). This latter expression, if it is used at all, would be taken to refer to the modern state of Vatican City, i.e., the enclave in Rome around St. Peter's Basilica over which the pope has temporal sovereignty (see further below).
The Western Roman Empire fell at the end of the fifth century CE, and in the sixth century, as the Byzantine emperors began to lose effective control of those parts of Italy over which they claimed authority, the pope gradually started to carry out some of their functions, first in the area around Rome and then in other parts of Italy. In 756 CE this situation was formally recognised in the so-called Donation of Pepin, when the Frankish king Pepin the Short (ruled 751-768), who had invaded Italy, defeated the Lombards, and taken control of northern Italy, granted the pope temporal jurisdiction over a broad band of territory stretching north east from Rome to Ravenna and thus brought the Papal States into existence. In 781 the territorial limits of the pope's temporal sovereignty were clarified by the Frankish king Charlemagne, who in 800 was crowned by pope Leo III as Emperor of the Romans (Augustus Romanorum (i.e., the first Holy Roman Emperor).
In the early centuries of the Papal States' existence the popes' ability to rule effectively was fragile and often subject to interference by Charlemagne's successors as Holy Roman Emperor. It was not until the beginning of the fourteenth century that the Papal States became a genuinely independent political entity, and even after that, during the period of the Avignon papacy (1307-1378), when the popes left Rome and took up residence in Avignon in the south of France, their ability to govern was severely limited, and powerful local families (e.g., the Malatesta in Rimini and the da Polenta in Ravenna) established themselves as rulers in many cities.
By the end of the fifteenth century the popes had succeeded in reasserting their authority over the Papal States, and especially during the papacies of Alexander VI (Rodrigo Borgia, reigned 1492-1503) and Julius II (Giuliano della Rovere, the 'Warrior Pope', reigned 1503-1513), an expansionist policy was pursued, the pope becoming one of Italy's most powerful secular rulers. (The Papal States are given a chapter to themselves in Il principe (The Prince, 1513), the celebrated work of the political philosopher Niccolò Machiavelli (1469-1527) - see Ch. 11 De principatibus ecclesiasticis (On Ecclesiatical Principalities).) The Papal States reached their greatest territorial extent in the eighteenth century, when they included the regions of Lazio, Umbria, the Marche, the cities of Ravenna, Ferrara, and Bologna, and other cities in the Romagna, small enclaves around the cities of Benevento and Pontecorvo in southern Italy, and even Avignon and its environs (acquired during the period of the Avignon papacy). Even then, though, effective government was often lacking and, as in much of the rest of Italy at the time, lawlessness and brigandage were rife.
During the Napoleonic era the Papal States were invaded twice by French armies, first in 1798, when a short-lived Roman Republic was established as a French client state, and secondly in 1808, when the Papal States were annexed to France as départements of the French Republic. After the fall of Napoleon the Papal States were restored, but the rise of Italian nationalism as well as other factors led to a growing mood of discontent: there were revolts in the Papal States, as in many other parts of Italy, in 1831; and in 1849, during the period when revolutions spread across Europe, another, equally short-lived, Roman Republic was declared in Rome after pope Pius IX had fled the city.
In 1860, after Garibaldi had defeated the Bourbon rulers of Sicily and southern Italy, Victor Emmanuel II, king of Piedmont and Sardinia, invaded, and assumed control of, the Papal States, which - apart from the city of Rome itself and its neighbourhood - were incorporated into the new nation of Italy. In 1870 Rome itself was taken and became the capital of the new national state. The pope refused to recognise the secular Italian state, and retreated to the area around St. Peter's Basilica on the west bank of the Tiber, where he was allowed to remain, though without any formal recognition of his temporal jurisdiction. (See further Garibaldi, Cavour, and Risorgimento.)
This situation persisted for nearly 60 years until 1929 when by the Lateran Treaty Italy recognised the temporal sovereignty of the pope over the area around St. Peter's Basilica, and the State of Vatican City came into existence.