Gregory, name of 16 popes. Saint Gregory I (540–604), called Gregory the Great, was pope from 590 to 604. His papacy laid the foundation for the political and moral authority of the medieval papacy. He reorganized the vast papal estates scattered all over Italy, providing an economic foundation for the Church's power. In 596 he sent St. Augustine to Britain, beginning its conversion to Christianity. Saint Gregory II (669–731), pope from 715 to 731, held office at a time of increasing conflict between Rome and Byzantium, and eventually excommunicated Patriarch Anastasius of Byzantium. Saint Gregory III (d. 741), pope from 731 to 741, continued to be involved in conflicts with Byzantium, excommunicating Byzantine Emperor Leo III. Saint Gregory VII (Hildebrand; c.1025–85), was pope from 1073 to 1085. One of the great medieval reform popes, he attacked corruption in the Church, and insisted on the celibacy of the clergy and on the sole right of the Church to appoint bishops and abbots. These reforms threatened the power of the German monarchy, leading to disputes and war with Henry IV of Germany. In 1084 Henry seized Rome, forcing Gregory to flee. Gregory IX (Ugolino; c.1170–1241) was pope from 1227 to 1241. His papacy was marked by conflict with Holy Roman Emperor Frederick II, leading eventually to war in Italy between imperial and papal factions. Gregory X (Tedaldo Visconti; 1210–76) pope from 1271 to 1276, instituted policies that regulated papal elections and prevented vacancies of long duration. Gregory XI (1329–78) was pope from 1370 to 1378. Elected pope in Avignon, he managed to return the papal court to Rome in 1377. Gregory XIII (Ugo Buoncompagni; 1502–85), pope from 1572 to 1585, promoted the Counter-Reformation through his pledge to execute the decrees of the Council of Trent. A patron of the Jesuits, he is remembered for the calendar reform he sponsored and for his lavish building program, which emptied the papal treasury. He celebrated the massacre of the Huguenots on St. Bartholomew's Day, 1572, with a Te Deum. Gregory XVI (1765–1846), pope from 1831 to 1846, strengthened the papacy, aligning it with Austria under Metternich, with whose help he suppressed a revolt in the Papal States.