Baldassarre Cossa was Pisan antipope John XXIII (1410–1415) during the Western Schism. The Catholic Church regards him as an antipope, as he opposed the Pope whom the Catholic Church now recognizes as the rightful successor of Saint Peter. He was eventually deposed and tried for various crimes, though later accounts question the veracity of those accusations.
Cardinal Cossa was one of the seven cardinals who, in May 1408, withdrew their allegiance from Pope Gregory XII, stating that he had broken his solemn oath not to create new cardinals without consulting them in advance. In company with those cardinals who had been following Antipope Benedict XIII of Avignon, they convened the Council of Pisa, of which Cossa became a leading figure. The aim of the council was to end the schism; to this end they deposed both Gregory XII and Benedict XIII and elected a new pope Alexander V in 1409. Gregory and Benedict ignored this decision, however, so that there were now three simultaneous claimants to the papacy.
Alexander V died soon after, and on 25 May 1410 Cossa was consecrated a bishop, taking the name John XXIII. He had become an ordained priest only one day earlier. John XXIII was acknowledged as pope by France, England, Bohemia, Portugal, parts of the Holy Roman Empire, and numerous Northern Italian city states, including Florence and Venice and the Patriarchate of Aquileia; however, the Avignon Pope Benedict XIII was regarded as pope by the Kingdoms of Aragon, Castile, Sicily and Scotland and Gregory XII was still favored by Ladislaus of Naples, Carlo I Malatesta, the princes of Bavaria, Louis III, Elector Palatine, and parts of Germany and Poland. John XXIII made the Medici Bank the bank of the papacy, contributing considerably to the family's wealth and prestige.
The main enemy of John was Ladislaus of Naples, who protected Gregory XII in Rome. Following his election as pope, John spent a year in Bologna and then joined forces with Louis II of Anjou to march against Ladislaus. An initial victory proved short-lived and Ladislaus retook Rome in May 1413, forcing John to flee to Florence. In Florence he met Sigismund, King of the Romans. Sigismund wanted to end the schism and urged John to call a general council. John did so with hesitation, at first trying to have the council held in Italy (rather than in a German Imperial City, as Sigismund wanted). The Council of Constance was convened on 30 October 1413. During the third session, rival Pope Gregory XII authorized the council as well. The council resolved that all three popes should abdicate and a new pope be elected.
In March, John escaped from Constance disguised as a postman. According to the Klingenberger Chronicle, written by a noble client of Frederick IV, Duke of Austria, John XXIII travelled down the Rhine to Schaffhausen in a boat, while Frederick accompanied him with a small band of men on horseback. There was a huge outcry in Constance when it was discovered that John had fled, and Sigismund was furious about this setback to his plans for ending the Schism. The King of the Romans issued orders to all the powers on the Upper Rhine and in Swabia stating that he had declared Frederick to be an outlaw and that his lands and possessions were forfeit. In due course this led to a great deal of political upheaval and many Austrian losses in the region, notably in Aargau to the Swiss Confederation.
In the meantime, Pope John XXIII and Frederick fled further downriver along the Rhine to the town of Freiburg im Breisgau, which recognised the duke of Austria as its lord. There Sigismund's lieutenant Ludwig III, Elector Palatine caught up with them. He convinced Frederick that he stood to lose too much by harbouring the fugitive pope, and the Austrian duke agreed to give himself and John up and return to Constance.
During his absence John was deposed by the council, and upon his return he was tried for heresy, simony, schism and immorality, and found guilty on all counts. Gibbon wrote, "The more scandalous charges were suppressed; the vicar of Christ was accused only of piracy, rape, sodomy, murder and incest." John was given over to Ludwig III, Elector Palatine, who imprisoned him for several months in Heidelberg and Mannheim. The last remaining claimant in Avignon, Benedict XIII, refused to resign and was excommunicated. Martin V was elected as new pope in 1417.
Cossa was again imprisoned in Germany. He was freed in 1418 after a heavy ransom was paid by the Medici. He went to Florence, where he submitted to Martin V, who made him Cardinal Bishop of Frascati. Cossa died only a few months later.