Cossa, Baldassare (bäldäs-säˈrā kôsˈsä) [key], c.1370–1419, Neapolitan churchman, antipope (1410–15; see Schism, Great) with the name John XXIII. He had a military career before entering the service of the church. He was made a cardinal by Boniface IX (1402) and proved himself able, especially in financial matters. In 1408 he deserted Gregory XII and helped to bring about the Council of Pisa to end the schism between the Roman and the Avignon popes. The council, declaring both Gregory XII and Benedict XIII deposed, set up a third claimant, Alexander V. On Alexander's death a year later, Cardinal Cossa was elected. Of the three rival "popes," John had by far the greatest following. He immediately sought the aid of Sigismund and helped elect Sigismund Holy Roman emperor. John allied himself with Louis II of Anjou (later king of Naples) to make war on Lancelot of Naples and his ally Gregory XII. An ineffective council at Rome (1412–13) was followed by the Council of Constance (see Constance, Council of), which John convened under pressure from Sigismund. At the opening of the council he reluctantly promised (1415) to abdicate if his rivals would do so. Then, surreptitiously, he fled to the lands of his ally Frederick of Hapsburg. He was forced to return. The council formally deposed him, and he submitted. He was held prisoner in Germany until released by Martin V in 1418; he returned to Italy. He died cardinal bishop of Tusculum. In his lifetime he had a reputation for unscrupulousness and self-aggrandizement.