Pius IV, 1499–1565, pope (1559–65), a Milanese named Giovanni Angelo de' Medici; successor of Paul IV. He was probably not related to the great Medici family. His career in Rome began in 1527, and he held increasingly important offices under Clement VII, Paul III (who made him a cardinal), and Julius III. Cardinal Medici was one of the reform party, but he was no rigorist, hence he was out of favor with Paul IV. The great feature of his pontificate was the reconvening of the Council of Trent (see Trent, Council of) for its last and most important session (1562–63). By quietly easing the difficulties of the council and publicly backing it, Pius gained new respect for the papacy and made himself one of the great popes of the Counter Reformation. He welcomed the final break with Protestantism, which the council brought about. His good political relations with Spain were in contrast with Paul IV's anti-Hapsburg policy. Pius's chief aid was his nephew, St. Charles Borromeo. He was succeeded by St. Pius V.
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