Pope from 1227. He excommunicated Frederick II for his delay in fulfilling his promise to go on a crusade, and when he sailed unreconciled in 1228, proclaimed an interdict over his land and wherever he went. In 1230 he agreed to a treaty with the Emperor, but in 1239 he excommunicated him again and died while Frederick was besieging Rome.
A friend of St Francis of Assisi, he became Protector of the Franciscan Order in 1220, and it was largely from Dominicans and Franciscans that he recruited the full-time Inquisitors appointed from c.1233. In 1230 he commissioned Raymond of Peñafort to collect the Papal decretals of the last 100 years in the so-called ‘Liber Extra’ (published in 1234).