Innocent III
Pope from 1198 until his death (c. 1160–1216), born Lotario dei Conti di Segni into a noble Roman family, trained in theology in Paris and in law in Bologna, and, for most historians, the most powerful pope of the entire Middle Ages, under whom the papacy reached the zenith of its temporal and spiritual authority over Latin Christendom. Elected pope at thirty-seven, he resolutely developed the doctrine of plenitudo potestatis, the fullness of papal power above kings and emperors, and intervened directly in the politics of almost every European kingdom: he excommunicated King John of England and forced him to accept England and Ireland as a papal fief, arbitrated the disputed German imperial succession between Guelphs and Ghibellines, and convened the Fourth Lateran Council (1215), the most important ecclesiastical gathering of the entire Middle Ages, which codified the doctrine of transubstantiation, required annual confession and communion of all the faithful, and thoroughly reformed church administration and the persecution of heresy. He promoted the Fourth Crusade, which ended up being tragically diverted into the sack of Constantinople in 1204 despite the pope's own belated and ineffective opposition, and the brutal Albigensian Crusade in southern France, which unleashed years of war and massacres; he also approved, not without initial wariness, the new mendicant orders of Francis of Assisi and Dominic de Guzmán, decisive for the Church's future. Before becoming pope he had written the ascetic treatise De miseria humanae conditionis ('On the Misery of the Human Condition'), a pessimistic and widely read medieval meditation on the fragility, corruption and decay of the body and the world, which contrasts vividly with the relentless and effective exercise of power that characterised his pontificate.