Second Council of Lyon in the context of "Pope Gregory X"

Stefan Uroš II Milutin (Serbian Cyrillic: Стефан Урош II Милутин, romanized: Stefan Uroš II Milutin; c. 1253 – 29 October 1321), known posthumously as "the Holy King" (Свети краљ), was the King of Serbia between 1282–1321, a member of the Nemanjić dynasty. He was one of the most powerful rulers of Serbia in the Middle Ages and one of the most prominent European monarchs of his time. Milutin is credited with strongly resisting the efforts of Byzantine Emperor Michael VIII Palaiologos to impose Roman Catholicism on the Balkans after the Union of Lyons in 1274. During his reign, Serbian economic power grew rapidly, mostly due to the development of mining. He founded Novo Brdo, which became an internationally important silver mining site. As most of the Nemanjić monarchs, he was proclaimed a saint by the Serbian Orthodox Church with a feast day on October 30.

The papal election that followed the death of Pope Clement IV lasted from 1 December 1268 to 1 September 1271 and was the longest in the history of the Catholic Church. This was due primarily to political infighting between the cardinals. The election of Teobaldo Visconti as Pope Gregory X was the first example of a papal election by "compromise", that is, by the appointment of a committee of six cardinals agreed to by the other ten. (This method was attempted once before, in the 1227 papal election, but the choice of the committee refused the honor and the full group of cardinals proceeded to elect the pope.) The election occurred more than a year after the magistrates of Viterbo locked the cardinals in, reduced their rations to bread and water, and removed the roof of the Palace of the Popes in Viterbo where the election took place.

As a result of the length of the election, during which three of the twenty cardinal-electors died and one resigned, Gregory X promulgated the papal bull Ubi periculum on 7 July 1274, during the Second Council of Lyon, establishing the papal conclave, whose rules were based on the tactics employed against the cardinals in Viterbo. The first election held under those rules is sometimes viewed as the first conclave.

Pope Martin IV (Latin: Martinus IV; born Simon de Brion; c. 1210/1220 – 28 March 1285), was the head of the Catholic Church and ruler of the Papal States from 22 February 1281 until his death in 1285. He was the last French pope to hold his court in Rome before the papacy moved to Avignon.

Before his election, Simon de Brion was a prominent French cleric who served as chancellor to Louis IX of France and was made a cardinal by Pope Urban IV in 1261. His papacy was marked by close dependence on Charles of Anjou, whom he appointed Senator of Rome, and by significant political conflicts, including the excommunication of the Byzantine Emperor Michael VIII Palaiologos, which ended the fragile union between the Eastern Orthodox and Roman Catholic Churches established at the Second Council of Lyons in 1274. Martin IV also faced the Sicilian Vespers uprising and excommunicated Peter III of Aragon, declaring a crusade against him in an unsuccessful attempt to maintain Angevin control over Sicily.