Lotario de' Conti di Segni was the son of Count Trasimund of Segni and nephew of Pope Clement III. His father was a member of a famous house that produced nine popes, including Gregory IX, Alexander IV and Innocent XIII. His mother, Claricia, belonged to the noble Roman family of Scotti.
Lotario was educated in Rome, Paris (under Peter of Corbeil), and Bologna (under Huguccio); he was considered an intellectual and one of the greatest canon lawyers of his time.
After the death of Pope Alexander III, Lotario returned to Rome and held office during the short reigns of Lucius III, Urban III, Gregory VIII, and Clement III, reaching the rank of Cardinal Deacon in 1190. 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.
On the day Celestine III was buried, Lotario was elected pope and took the name of Innocent III. He was just thirty-seven years of age, and although a deacon, not yet a priest. Throughout his career as Pope, Innocent sought to reassert and extend the plenitudo potestatis (the secular power) of the holy see. 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 took advantage of the confusion to lessen imperial (German) influence in Italy; his first act was the restoration of the papal power in Rome. The Prefect of Rome, who reigned over the city as the emperor's representative, swore allegiance to Innocent. Innocent demanded the restoration to the Church of the Romagna and the March of Ancona from Markwald of Anweiler and used papal troops to bring this about. In a similar way, the Duchies of Spoleto, Assisi and the Sora were taken from the German Conrad von Uerslingen.
The pope also made use of the weakness of King Frederick of Sicily (who was only four years old) to reassert papal power in Sicily; he 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. The pope then invested the young Frederick as King of Sicily in November, 1198. He also later induced Frederick to marry the widow of King Emeric of Hungary in 1209.
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. In 1201 the pope openly supported Otto, threatening with excommunication all those who refused to acknowledge him. By the decree Venerabilem in May 1202 Innocent 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 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.
In 1207, Innocent 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, and at the Diet of Frankfurt of November 11, 1208, Otto was acknowledged as king. The pope invited him to Rome and he was crowned emperor as Otto IV in Rome on October 4, 1209.
Prior to his coronation, Otto had 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 (the stipulation of Neuss, which promise he repeated at Speyer in 1209). But soon after he being crowned, Otto seized Ancona, Spoleto and other territories claimed by the Church, giving them to certain of his vassals. He also invaded the Kingdom of Sicily. As a result, Otto 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 of Sicily. Frederick 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, he was crowned Frederick II at Aachen on July 12, 1215.
Innocent had forced Philip Augustus of France to be reconciled with his wife, Ingeborg of Denmark, and Philip Augustus became, thereby, Innocent's ally in the struggle over Otto. Otto allied himself with England (he was nephew of King John 'Lackland') to fight Philip Augustus, but he was defeated in the Battle of Bouvines in what is now Belgium, on July 27, 1214. Thereafter Otto lost all influence and died on May 19, 1218, leaving Frederick II the undisputed emperor. Innocent 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, which was opposed by King John. The king was forced to acknowledge the Pope as his feudal lord and accept Langton.
Innocent was a strenuous opponent of heresy. He had the Papal States cleared of the Manichean heretics and instigated a campaign under the leadership of Simon de Montfort against the Albigenses (Cathars), the Albigensian Crusade (1209-1229). This was a prelude to the legitimisation of the Inquisition in 1233, wherein heresy was to be punished for the spiritual good of the individual as well as for the preservation of the Church. The Pope supported two new holy orders: the Franciscans and the Dominicans.
Innocent had also decreed the Fourth Crusade in 1198, which was intended to recapture the Holy Land. The pope directed his call towards the knights and nobles of Europe rather than to the kings; he wished that neither Richard I of England nor Philip II of France, who were still engaged in war, not to mention his German enemies, should participate in the crusade. Innocent's call was generally ignored until 1200, when a crusade was finally organized in Champagne. The Venetians then re-directed it into the sacking of Zara in 1202 and of Constantinople in 1204. Innocent was not pleased with the means by which it was done, but he accepted the Sack of Constantinople because it brought about the temporary reunification of the Catholic and Orthodox churches after the Great Schism of 1054. He did excommunicate the Venetians he held to be responsible.
Innocent also summoned the Fourth Lateran Council (12th ecumenical council) in November 1215. It decided on another crusade to the Holy Land (the Fifth Crusade) and issued some seventy reformatory decrees. Among other things, it encouraged creating schools and holding clergy to a higher standard than the laity.
Innocent III died at Perugia and was buried in the cathedral there, where his body remained until Pope Leo XIII had it transferred to the Lateran in December 1891. Although the papal power over kings that Innocent III established would be short-lived, he sincerely attempted to turn theological principles into actual powers. Two of his Latin works are still widely read: De Miseria Humanae Conditionis, a tract on asceticism that Innocent wrote before becoming Pope, and De Sacro Altaris Mysterio, which is a description and exegesis of the liturgy.