The last time the pope retired was in 1415, when Gregory XII resigned to try and resolve the Western Schism. Now, there was partisanship for you, partisanship which makes our divided politics look like a scuffle in the park. At one point, there were three people claiming to be the Pope in the west, one based in Rome, one in Avignon, and one in Pisa. The period of the Avignon Papacy was started, as most things are, by secular politics. The Pope of the time, Boniface VIII , and the French King, Philip IV, had a running feud centering around the limits of papal power in France. Philip thought there were rather a lot of limits; Boniface did not agree.
This led to the issuance, in 1302, of the Bull Unam Sanctam, which asserted the Church’s authority over all kingdoms temporal (the quote in the title comes from it). Philip did not take Boniface’s statement lightly and during the Pope’s vacation at the Italian hill town of Anagni in September 1303, a force of French-hired mercenaries marched into town and took the Pope captive. One of the commanders of the force, Sciarra Colonna, is reputed to have slapped the Pope. The “Outrage of Anagni” lasted for three days, with the Pope held captive by the French. Then the townspeople of Anagni, irate at the violation of their pontiff, rose up, freed the Pope, and evicted the French. Boniface VIII, though free, was so shaken by the incident that he up and died about a month later.
His replacement, Benedict XI, lifted Philip IV’s excommunication and for the most part ignored the claims of Unam Sanctam. He did excommunicate those directly involved with the Outrage of Anagni, however, and when he died suddenly after eight months, there was some thought he had been poisoned. His successor, Clement V, was French, and rather than coming to Rome to take up the Papacy settled instead in Avignon, in the south of France.
There the popes remained, for nearly a century, under the thumb of the French kings. Whatever the confusion of the situation, however, at least there was one officially-recognized Pope. That would change. In 1377, Gregory XI returned to Rome from Avignon for good (his predecessor, Urban V, had tried to come back, but failed). When Gregory died, the College of Cardinals elected Urban VI Pope. They soon regretted their choice, as Urban proved capricious and ill-tempered. As a result, in the same year, the cardinals fled to Anagni (hello, Anagni! Nice to see you again), and elected as Popeanother one of their members. He became Clement VII, and headed to Avignon where he established his Papacy.
This naturally caused chaos. The College of Cardinals had elected two popes, and there wasn’t really a way to figure out who was the legitimate one–except by appealing to the Pope to rule on it. You can imagine how each Pope ruled on that question. The two Popes, spitting invective at each other, remained (in the form of different people) in Rome and Avignon for the rest of the 14th century. Early on in the 15th century, a council was arranged at Pisa to settle the schism, but the two popes refused to show up. The cardinals from both sides nonetheless met to try and resolve it. Rather than doing that, however, they elected ANOTHER Pope, Alexander V.
Oy. Reports that at this point the cardinals began using a scorecard to track the number of Popes could not be confirmed.
In the end, the whole fine mess was resolved by the Council of Constance, in 1414, which managed to get both the Pisan Pope and the Roman Pope to resign, and then chose a single successor to unify the two lines. The Avignon Pope, Benedict XIII, refused to go along and was promptly excommunicated (well, three years later, but that’s promptly for the Middle Ages). The Avignon line (no longer in Avignon but in Aragon) lasted for several more decades in increasingly weird ways (check out the note about the “hidden pope.”) and then faded away.
The Roman line is, of course, still going, and they have a successor to choose.