The international situation of the time along with his political capabilities.

Lotario dei Segni was born around 1160 into an aristocratic family of Latium, the region where Rome is. His contemporaries noted him for his extraordinary memory and formidable intelligence. He received the best education of the time, studying Theology at the university of Paris and Law at the university of Bologna. He was made a cardinal around 1190 and his books became very popular.

When he was made Pope Innocent III in 1198, he was 38.

The papal states in central italy risked to be encircled by the possessions of the House of Staufen. The marriage of Constance of Sicily to the HRE Emperor Henry VI had produced a boy, Frederick II, who was therefore heir of the Sicilian kingdom (all of southern Italy) and the future likely candidate to the Imperial throne (with sovereignity over Germany + northern Italy).

In order to not become an enemy of the papacy and to have the Pope accept this unio Regni ad Imperii (union of the kingdom and of the empire), Constance appointed Innocent III protector of the young Frederick II. When Constance died (Henry VI was already dead), Innocent III turned the kingdom of Sicily (formally the inheritance of Frederick II) into a papal fief. He also gave the Holy Roman Imperial crown to Otto IV (it was a papal right to appoint someone else as Emperor after an election was held by the German princes, given that the Empire was not hereditary) and made sure the crowns of Sicily and HRE remained separated. Both the Emperor and the King of Sicily depended on Innocent III for their position, and the independence of the Papal States was guaranteed.

Innocent III used the prestige he accumulated to score more diplomatic victories and spread his propaganda. About 10.000 letters were sent to monarchs, intellectuals, and clergymen by Innocent III (or in his name) during his lifetime. He famously argued that the "Pope is at center of the world, below God, but above men". In a very Christian world, with a Church so influential and now so politically successful, this message resonated with commoners and monarchs alike. Norway, Hungary, Aragon, and England sent tribute to Innocent III for his offices. Those kingdoms did so also because they were afraid of other powers (Aragon was afraid of France and Castile, the King of England was afraid of his own barons after issuing the Magna Charta, Hungary was afraid of the Empire etc). Having the universal pastor of every christian as a guardian was an effective political strategy.

He also used the instrument of the crusade to further establish his authority. For example, he made himself arbiter in the wars fought between the christian Spanish kingdoms and directed their efforts against the moors (Battle of Las Navas) by launching a crusade. He launched the fourth crusade, of which he lost control, but the results (Byzantium turned into a Latin empire) diminished Orthodox prestige and increased Catholic presence in the west.

When Otto IV decided he had enough of Papal supremacy, he invaded Italy. However, the Pope convinced a part of the German electors vote for Frederick II as Emperor in Otto's place. Innocent III used an alliance he had with the King of France (again, for a crusade, in this case a crusade against heretics in south france) to launch a war against Otto IV. Together, Philip II of France and Frederick II deposed Otto IV (1214-5). Frederick II promised to abdicate as king of sicily now that he was Emperor. However, Innocent III died (1216) and the Popes after him were encircled by Frederick II (who broke his promise, and remained both King of Sicily and Holy Roman Emperor).

Sources: Innocenzo III by Werner Maleczek, 2000, Treccani Editore,