From the perspective of worldly power, Innocent III was the most successful of the popes. He was a sincere and competent individual, but he shows that these traits can be very dangerous when used in the wrong way. He held to the absolute authority and infallibility of the pope, giving him unquestioned rule over all ecclesiastical and civil powers.
He increased the pope's power over the areas of Italy surrounding Rome and worked to humble kings, using political maneuvering and the interdict (denying to nations the priestly services until their rulers submitted). When John Lackland of England tried to force his own choice for archbishop of Canterbury over the one elected by the local church leaders, Innocent annulled both choices and put in his own candidate. The pope was no longer defending the local church leaders against political encroachment, but was imposing his will on everybody. Later, after John submitted to Innocent but the barons rebelled against John, forcing on him the Magna Carta, Innocent absolved John from having to obey it. Thus Innocent defended absolute rule over the attempt to deal with genuine grievances. Also, when the fourth crusade took Constantinople, Innocent, while initially deploring the act, ended up endorsing it in the hopes of reuniting the eastern and western churches. But in the long run, it ended up greatly alienating the eastern church.
The Albigensians, who were mainly Mancheans who believed there were two Gods, a good and an evil God, but who may have included other elements in rebellion against the established church, were multiplying in areas of France. When efforts at persuasion failed, Innocent called a crusade against those areas affected. He also created the Inquisition to deal with heresy in a systematic way. While the Medieval justice system was not very just at any time, including trial by ordeal and by combat, it was arbitrary to the advantage of either side in the dispute. But the Inquisition was clearly on the side of the accuser, allowing anonymous accusations and examination by torture, procedures taken up by other courts. Innocent also officially affirmed transubstantiation as the required belief, that Christ was physically present in the Lord's Supper, making reconciliation with some of the rebel groups more difficult. He also approved the creation of the Franciscan and Dominican orders (of which I will have more to say in a later post), which became very useful to the papacy.
All this use of strong-arm tactics created a reaction which other, less powerful popes had trouble standing against. The use by the pope of political clout made people cynical of the spiritual pretensions of the office. Also, while Innocent tried to correct some of the clerical abuses common in his time, setting the pope above question helped lay the foundation for future abuses and made them difficult to cure. And the attempt to suppress all rebellion against the papacy failed in the long run. The attempt to solve spiritual problems with political power ultimately proved futile. And it still is.