While the Vatican City is today regarded as center of Roman Catholicism, for over 1,000 years, until about the late 13th century, the Pope actually resided in the Lateran Palace on the other side of what is now the Italian capital. Generally speaking, the Pope is nowadays only a spiritual figure, overseeing matters related exclusively to the functioning of the Catholic Church. But during the middle-ages, Popes where also legitimate heads-of-states and ruled a considerable amount of lands throughout the Italian Peninsula, known then as the Papal States. Popes of this time exacted both spiritual and secular influence across the political landscape, often making proclamations in their capacity as leader of the Church in order to make diplomatic or economic gains as rulers of their territories.
Ultimately, this incited conflict with other powers and political organizations throughout the continent. Most notably, in 1302, Pope Boniface VIII made an order to excommunicate King Phillip IV of France after the French king denounced a proclamation of 'Papal Supremacy' made by the Vatican. This order of excommunication roused French sympathizers in Italy, who subsequently broke into the Pope's residence and assassinated Boniface. While it seemed like the Papal States were on the brink of war with France, subsequent Popes began to deescalate tensions with Philip, and eventually his own friend, Clement V, was elected as Supreme Pontiff.
This pro-French movement within the Vatican caused distress with the other European powers, especially the monarchs of England and the Holy Roman Empire in Central Europe. More immediately, it led to a fracturing of relations with many leaders throughout Italy and within Rome itself. This diplomatic crisis, coupled with pressure from the King Philip who was seeking more influence over Church, led Pope Clement V to move the administration of both the Church and the Papal States from Rome to Avignon. And so, from March 9th, 1309 until January 13th, 1377, the Pope officially remained along the banks of the Rhône, and the Catholic Church entered a period of intense Franco influence, with seven subsequent French popes leading a church with a majority of French cardinals. Avignon, of course, would never be the same.