The Kingdom had its origins in the Wambid Kingdom of Italy, a division of the Western Roman Empire which came under the hereditary rule of the Florentian branch of the Wambid Dynasty. In those days it was common for younger sons to enter the priesthood so as not to be a rival to their brothers. Some rose quite high in the church hierarchy, and in 733 Bishop Agila of Florence, the seventh son of King Reccared, was elected by the College of Cardinals and took the name of Pope Stephen II.
With six older brothers before him in the line of succession, Stephen had never entertained any dynastic ambitions. However, during the Christmas feast of 735, a freak storm caused the roof of his father's feasting hall to collapse, killing his father, all his brothers and all their sons, as well as many other great lords of the realm. Stephen was left as the last living male of his house, as well as the last person in the country who could possibly take control, and therefore in the new year he was brought before the royal seat in Ravenna and acclaimed King of Italy.
When Stephen grew old the question of succession arose once again. With no children of his own, he knew his death would mark the extinction of the House of Florence, and he knew that none of the temporal lords of the realm were powerful enough to bring order to the country. Therefore in 757 he issued a papal bull, turning the Kingdom into a papal possession for all time, to be passed from pope to pope and never again entering secular hands.
Pope-King Stephen died in 758 and, despite some initial resistance, Italy was indeed passed on to the new Pope-King Paul I. From then on the kingship and the Papacy were one and indivisible,
Fall of the Kingdom
The Kingdom of Africa, another successor to the Wambid Empire, controlled Sicily and, intermittently, parts of southern Italy. In 867 King Theodemar II of Africa became Roman Emperor, restoring Roman influence in Italy.
In 1082, frustrated by constant raids across the border, the Roman governor in Naples received permission to invade the Papal Kingdom. By deposing Pope-King John XIII and installing a Greek Muslim in his place, the Empire hoped to indirectly control the Papacy and thus have great influence in western Europe. However, John managed to escape and created a new base in Milan, where for the rest of his life he spent his time issuing propaganda designed to rouse the West against the Roman heretics.
In 1096 Pope Urban II persuaded the princes of Europe to come together and overthrow the Romans by force in what he called a Great Crusade. Despite some initial success in Italy, the Crusade met with a disastrous naval defeat at Paphos, upon which its surviving leaders deserted en masse and returned to their own lands. The Papal Kingdom, despite regaining Rome itself, was left almost defenceless, with an angry and very powerful enemy on its southern border eager for revenge.
In 1104 Emperor Alexios Komnenos invaded Italy with an enormous army and navy in a three-pronged attack, swiftly capturing everything south of the Po. The new Pope-King Paschal II was arrested and exiled to Patmos, where he died in 1118, and the entire southern half of the Papal Kingdom was annexed into Romania proper. The north technically remained within the Holy Roman Empire, but without any proper central authority it soon disintegrated into a cluster of semi-independent city-states, some of which would persist for centuries.
A year later the surviving members of the College of Cardinals gathered in Mainz where they elected Benedict X as a new Pope. However, the Papacy never regained any temporal power and, as Islam continued to gain converts throughout Europe, it became increasingly irrelevant until it was finally abolished at the Council of Bergen in 1962.