Pope Innocent III (c. 1161 – June 16, 1216), born Lotario de' Conti di Segni, was pope from January 8, 1198 until his death.

Biography
Lotario de' Conti di Segni was born in Gavignano, near Anagni. His father was Count Trasimund of Segni and was a member of a famous house that produced nine popes, including Pope Gregory IX (1227–1241), Pope Alexander IV (1254–1261) and Pope Innocent XIII (1721–1724). His uncle was Pope Clement III (1187–1191), and his mother, Claricia, belonged to the noble Roman family of Scotti.
Lotario studied in Rome, Paris (theology, under Peter of Corbeil), and Bologna (canon law, under Huguccio). The latter's moderate doctrine on the relationship between spiritual and lay authorities were a constant influence in the future work of Innocent. He was considered an intellectual and one of the greatest canon lawyers of his time.
After the death of Pope Alexander III (1159–81), Lotario returned to Rome and held office during the short reigns of Lucius III (1181–1185), Urban III (1185–1187), Gregory VIII (1187), and Clement III (1187–1191, possibly a relative of the Segni), reaching the rank of Cardinal Deacon through his uncle Pope Clement III. During the reign of Pope Celestine III (1191–1198), a member of the House of Orsini, who were enemies of his family, Lotario left Rome to live in Anagni. During this period he wrote a series of theological works, including On the Miserable Condition of Man and On the Mysteries of the Mass, both showing the ascetic-liturgical inspiration animating him.
On January 8, 1198, the day Celestine III was buried, Lotario was unanimously elected pope after only two ballots. His election was held in the ruins of the ancient Septizodium, near the Circus Maximus in Rome and is is considered by some scholars as the first conclave. He took the name of Innocent III. He was only thirty-seven years old at the time. He was ordained a priest on February 21 and consecrated bishop of Rome the following day.

Early life and election to the Papacy
An energetic personality, throughout all his career as Pope Innocent III sought to assert and extend the prestige and plenitudo potestatis (plenitude of power) of the papacy, following the path of the process started by Gregory VII in the late 11th century, and which had diminished during the reign of his predecessors.
Even before his coronation, he thwarted the power of the aristocracy of Rome by obliging the senators of the city to swear allegiance to the popes. The Prefect of Rome, who reigned over the city as the Emperor's representative, had to do the same. Innocent reigned in Rome with the support of the oligarchic class, until a popular uprising in 1203, backed by the Orsinis, forced him to move to Palestrina. He returned to the pacified city the following year.
On the foreign side, the throne of the Holy Roman Empire had become vacant by the death of Henry VI in 1197, and no successor had yet been elected. Innocent III took advantage of the confusion to lessen imperial (German) influence in Italy. The pope demanded the restoration to the Church of the Romagna, the March of Ancona, and the Duchy of Spoleto from imperial vicar Markward of Anweiler; he used papal troops to bring this about, but failed to regain Romagna. In a similar way, the Duchies of Spoleto, Assisi and the Sora were taken from the German Conrad von Uerslingen. Innocent could not, however, regain the former possessions of Mathilda of Canossa in Tuscany.
The pope also made use of the weakness of Henry's son, King Frederick II of Sicily (who was only four years old), to reassert papal power in Sicily. Taking advantage of the last will of Frederick's mother, Constance of Sicily, which had named him as tutor of the young king, Innocent acknowledged Frederick as king only after the surrender of the privileges of the Four Chapters, which William I of Sicily had previously extorted from Pope Adrian IV (1154–59). The Pope then invested the young Frederick II as King of Sicily in November 1198. He also later induced Frederick II to marry the widow of King Emeric of Hungary in 1209.
However, not all his temporal affairs were successful. When acting as the guardian to the young Frederick II of Sicily he derailed much of the carefully constructed government created by the Norman kings. This reflects what Barraclough calls the "failure to consider the executive" — that is, the inability to exert sufficient temporal authority over governed areas.

Reassertion of Papal power
After the death of the Holy Roman Emperor Henry VI in 1197, two princely parties had elected competing kings: Philip of Swabia of the Hohenstaufen family, and Otto of Brunswick of the Welf family. Since Philip had been excommunicated by Celestine III and not crowned in Aachen, in 1201 the pope openly supported Otto; he threatened with excommunication all those who refused to acknowledge him. By the decree Venerabilem in May 1202 Innocent III made clear to the German princes his view of the relationship between the Empire and the papacy (this decree was afterwards embodied in the Corpus Juris Canonici). The decree asserted the papal rights to decide whether a king is worthy of the imperial crown and to arbitrate or to pronounce in favour of one of the claimants in case of a double election, as was the current situation with the Empire. He argued this bull on the grounds that the transition of the Roman Empire from Byzantium to the Holy Roman Emperor had taken place only under papal blessing, and therefore all blessing, coronation, and investiture of the emperor was dependent upon the pope.
Philip, however, gained increasing steam at the expense of Otto, and in 1205 received a more regular coronation at Aachen from the Archbishop of Cologne, Germany's main religious authority. Considering Otto the losing party, in 1207 Innocent III changed his mind and declared in favour of Philip, sending cardinals to Germany to induce Otto to renounce his claims to the throne. But Philip was murdered on June 21, 1208 (probably by Otto's agents), and, at the Diet of Frankfurt of November 11, 1208, Otto was acknowledged as emperor. The pope invited him to Rome and the two met at Viterbo, with Otto swearing to renounce to any claim to the Mathilda's heritage and the former exarchate of Ravenna (Romagna). He was then crowned as Emperor Otto IV, in St. Peter's Basilica, on October 4, 1209.
Otto IV had also promised to leave the Church in possession of Spoleto and Ancona and to grant the freedom of ecclesiastical elections, unlimited right of appeal to the Pope, and the exclusive competency of the hierarchy in spiritual matters. He had also promised to assist in the destruction of heresy (in what is known as the stipulation of Neuss, a promise that he repeated at Speyer in 1209). But soon after being crowned, Otto IV seized Ancona, Spoleto, and other territories claimed by the Church, giving them to his vassals. He also invaded the Kingdom of Sicily. As a result, Otto IV was excommunicated on November 18, 1210.
At the Diet of Nuremberg in September 1211, the pope convinced some imperial princes to renounce the excommunicated emperor and to elect Frederick II of Sicily. Frederick II made the same promises as Otto IV had done; he was reelected by most of the princes on December 5, 1212, and, his election being ratified by Innocent III, he was crowned at Aachen on July 12, 1215.

Encroachment in Empire's affairs
Innocent's personal strength and personality made him the most prominent political figure in Europe: he had John "Lackland" of England declare himself vassal of the Church (1213); received the feudal homage of Peter II of Aragon, Ottokar I of Bohemia, Alfonso IX of Leon and Sancho I of Portugal; and forced Philip II Augustus of France (1180–1223) to be reconciled with his wife, Ingeborg of Denmark. Philip II thereby became Innocent III's ally in the struggle over Otto IV. Otto allied himself with England (he was the nephew of John "Lackland") to fight Philip II Augustus, but he was defeated in the Battle of Bouvines in what is now Belgium, on July 27, 1214. Thereafter Otto IV lost all influence and died on May 19, 1218, leaving Frederick II the undisputed emperor. Innocent III played further roles in the politics of France, Sweden, Bulgaria, Spain, and especially England.
In England, there was controversy over the appointment of Stephen Langton as Archbishop of Canterbury, a decision that had been made in Rome (without consultation) by Innocent himself and which was opposed by King John and by the majority of the monks of Canterbury Cathedral. The king was eventually forced to acknowledge the pope as his feudal lord and accept Langton, after Innocent stirred up John's former enemy, the French king, to invade England. Innocent also declared the Magna Carta invalid at King John's request, on the grounds that it had been obtained by force and that as John was the pope's feudal vassal he was unable to enter into binding contracts of this nature without papal permission. This papal tampering in the internal affairs of a sovereign state was to have significant consequences later in English history: at the time of the Henrician Reformation in the early sixteenth century this case was cited by the king's men of law as evidence of unwarranted papal interference in English affairs and helped to bolster the popular case for casting off Rome.
Innocent intervened regularly in the affairs of Sardinia, sometimes at the invitation of the local giudicati and sometimes as part of his own agendum. At the beginning of his pontificate, he recognized the suzerainty of the Archdiocese of Pisa over Sardinia. Innocent intervened in the wars between the Giudicato of Cagliari and the Giudicato of Logudoro to establish a peace and tried to sort out the accusations William I of Cagliari and Comita III of Torres levelled at one another. He ordered the island prelates to investigate the legality of the marriages of the giudici (probably to gather ammunition against them if necessary) and even called William and Comita to Rome, but the Republic of Pisa, of which they were both citizens, refused to allow them to appear before a "foreign" tribunal. This sparked a conflict with Pisa. Innocent threatened to deprive the Pisan Archbishop Ubaldo of his legatine rights on the basis that "he who abuses his power, deserves to lose his privilege." Innocent tried to extract an oath of homage from William to the Holy See, but the Pisan archbishop refused to absolve William from previous oaths to himself. Innocent also tried to verify the accusations made against Giusto, Archbishop of Arborea, who had been removed from his see by Ubaldo and William, but failed to have him reinstated.
In 1202, when the Archdiocese of Torres became vacant, Innocent appointed a member of his own curia, Biagio, archbishop to carry out his personal orders on the island. In 1203, Barisone II of Gallura died, leaving his widow and heiress, Elena, in the care of Innocent, who charged the other giudici with her protection and gave Biagio the job of finding her a suitable marriage. The pope tried to arrange a marriage with his relative Trasimondo, but Elena rebuffed this attempt and instead married a Pisan, Lamberto Visconti. Innocent's policies in Sardinia were stiffly opposed and when he died the island was under Pisan hegemony.