As early as the 4th century, the popes had acquired considerable property around Rome (called the Patrimony of St. Peter). From the 5th century, with the breakdown of Roman imperial authority in the West, the popes’ influence in central Italy increased as the people of the area relied on them for protection against barbarian invasions. Leo I (reigned 440–461), for example, prevented Attila the Hun from sacking Rome, and Gregory I (590–604) faced threats from the Lombards. Gregory reorganized the papacy’s vast estates and improved its administration of a charity. Notwithstanding these early developments, the papacy and its territories remained part of the Eastern Roman Empire (Byzantine Empire) during this period.
The Republic of St. Peter, or the Papal States, emerged in the mid-8th century as part of a broader political reconfiguration. Pope Gregory II (715–731) and Gregory III (731–741) turned away from the Byzantine Empire because of increased imperial taxation, the emperor’s policy of iconoclasm (prohibition of the veneration of religious images), and Constantinople’s failure to protect Rome adequately. When the Lombards threatened to take over the whole peninsula in the 750s, Pope Stephen II (or III; 752–757) appealed for aid to the Frankish ruler Pippin III (the Short), who “restored” the lands of central Italy to the Roman see, ignoring the claim of the Byzantine Empire to sovereignty there. This Donation of Pippin (756) provided the basis for the papal claim to temporal power. In the same year, by the Treaty of Pavia, the Lombard king Aistulf ceded territory in northern and central Italy. It was probably also about this time that the Donation of Constantine was forged by an unknown cleric in Rome. A legitimate donation by Charlemagne and decrees by Louis the Pious and his son Lothar I confirmed and expanded early Carolingian grants of territory to the papacy. The pope consequently became ruler of the area around Ravenna, the Pentapolis (along the Adriatic Sea from Rimini to Ancona), and the Roman region.
During the rest of the Middle Ages, the popes were able to maintain their sovereignty over this territory despite changes in the political landscape. After the breakup of the Carolingian empire in the 9th and 10th centuries, the papacy came under the control of the local Roman nobility. Although not particularly effective as spiritual leaders, the nobles sought to preserve the papal territories. A greater challenge was posed by a conflict between the popes and the German Holy Roman emperors that began with the Investiture Controversy (1078–1122) and continued intermittently until the mid-13th century. The difficult relations with the emperors were exacerbated by a controversy over the lands of the countess Matilda of Tuscany, which she had initially donated (1102) to the papacy but finally left (1111) to the emperor Henry V.
In the 12th and 13th centuries, the popes faced the rise of commune governments, especially in the Romagna. Although generally supportive of the northern Italian communal movements, the popes opposed those in central Italy and in Rome itself, where a revolt against papal authority took place in the early 1150s. Despite such threats to the integrity of the Papal States, the papacy managed to expand its territories during this period. By an alliance with the Normans in the late 11th century, the duchy of Benevento was acquired in 1077. Pope Innocent III (1198–1216) took advantage of the dispute between the Hohenstaufen and their rival Otto IV for the imperial crown to promote his claims, notably in the march of Ancona; and in 1201 Otto acknowledged the church’s right to the duchy of Spoleto.
With the transfer of the papal residence from Rome to Avignon (1309–77), papal authority over the Italian territories became increasingly tenuous. The popes faced challenges from the Visconti of Milan and from the city of Florence, and papal representatives from Avignon were rejected by Bologna and other territories. Following a struggle between the noble Orsini and Colonna families for control of Rome, in 1347 Cola di Rienzo established a short-lived republic in the city. In an effort to reassert their authority, the popes turned to trusted military leaders such as Gil Cardinal Álvarez Carrillo de Albornoz, who reconquered the entire Papal States in a 10-year campaign (1353–63). Later, Robert of Geneva, the future antipope Clement (VII; 1378–94), undertook a series of diplomatic initiatives that paved the way for the return of the papacy to Rome. These successes, however, were undermined by the Great Schism (1378–1417), during which rival popes ruled from Avignon and Rome; in 1409 a third pope was elected at the Council of Pisa. The schism was finally ended at the Council of Constance, where the rival popes were deposed and Martin V (1417–31) was elected.
