In 751 AD, Rome was sieged by the Lombards. Previously, the city had been part of the Byzantine Empire. In 756 AD, Pepin the Short, King of the Franks, invaded Italy, freeing Rome from the Lombards and giving large regions of Italy to the Pope. This is how the Papal States arouse and gave the papacy a power it had not yet had.
The spread of Christianity gave the Bishop of Rome great religious and political strength and Rome became the center of Christianity. Until Rome was declared capital of the Kingdom of Italy, Rome was the capital of the Papal States.
Until the nineteenth century, the papacy was in a constant struggle with the Holy Roman Empire and other powers in Europe. Nonetheless, Rome became a very powerful and wealthy city, with a great international importance.
Factors that contributed to Rome becoming such an important place during the Papacy include: first, it became the most important pilgrimage site during the Middle Ages, secondly, in 1300 Pope Boniface VIII proclaimed the first Holy Year (Jubilee), a year of great importance for Christians; thirdly, the councils; fourth, the Papal patronage became focus of rebirth replacing Florence and finally the cultural influence of the city itself.
The papal authority continued to be extremely strong until the nineteenth century.
The end of papal authority
The French Revolution put an end to the papal power. With the 1848 revolution, Rome became part of the unified Italy, and after the battle of Porta Pia in 1870 (battle led by Pius IX to main his sovereignty over the Papal States) Rome was declared the new capital of Italy.