Background
John was the third son of Casimir IV Jagiellon, King of Poland, and Elisabeth Habsburg of Hungary, daughter of the King Albert of Hungary. In 1492, John succeeded his father as King of Poland thanks to the key intervention of his brother Friedrich Jagiellon, archbishop of Krakow and archbishop of Gniezno.
Career
As crown prince, he distinguished himself by his brilliant victory over the Tatars at Kopersztyn (1487). In 1490, the Hungarian nobility proclaimed John King of Hungary at the Rákos diet. However, losses of revenue due to the secession of Lithuania placed John at the mercy of the Polish sejmiks, or local diets, where the szlachta, or local nobles, made their subsidies dependent on the king"s subservience.
Circumstances seemed, moreover, to favor him.
As suzerain of Moldavia, John was favorably situated for attacking the Turks. At the conference of Leutschau (1494), the details of the expedition were arranged between the kings of Poland and Hungary and Elector John Cicero of Brandenburg, with the co-operation of Stephen III of Moldavia, hospodar of Moldavia, who had appealed to John for assistance.
Whatever the reason, the Poles entered Moldavia not as friends but as foes, and after the abortive siege of Suceava were compelled to retreat following defeat at the Battle of the Cosmin Forest. The insubordination of the szlachta seems to have been one cause of this disgraceful collapse, for John after his return confiscated hundreds of their estates.
In spite of which, to the end of his life he retained his extraordinary popularity.
His intention to still further humiliate the Teutonic Order was stymied by his sudden death in 1501.