After the death of Wladyslaw II Jagiello in June 1434, his son, Wladyslaw III, almost ten years old, succeeded to the throne with the agreement of the nobility. Government was carried out by a regency headed by a strong political individual, Zbigniew, bishop of Cracow, with a royal council still formed mainly of nobles from Little Poland. The opposition signified its dissatisfaction with the regency and made contact with the Bohemian Hussites. From that quarter there came, after the death of the Emperor Sigismund of Luxemburg in 1437, the proposal that Wladyslaw II’s younger son, Casimir, should take the Bohemian crown. The royal widow, Sophia, supported the plan but Zbigniew objected to it. Despite Polish attacks on Bohemia and Silesia, the Polish candidate lost to Albert of Habsburg. The bishop-regent helped Grand Duke Sigismund consolidate his position in Lithuania and defeat opposition from Svitrigaila. In Little Poland, Zbigniew fought the Hussite opposition led by Spytko of Melsztyn which enjoyed noble and even peasant support. In 1439 at the battle of Grotniki on the Nida the revolt was put down. Spytko was slain and denounced as an enemy of the country.
After the assassination of Grand Duke Sigismund at the hands of his boyars in 1440, the young Casimir, then thirteen years old, was sent to Lithuania in Wladyslaw III’s name as royal lieutenant. The Lithuanian nobles immediately acclaimed him as grand duke and renounced the personal union with the Polish crown. Without abandoning hope of renewing the Lithuanian alliance, the Polish nobles sought compensation in a union with Hungary.
In 1439, following the death of Albert of Habsburg who had been king of Bohemia and Hungary for a short time, the Hungarian nobles, in the face of the Turkish threat, turned to the court of Cracow and offered the crown of St Stephen to Wladyslaw III and, despite pro-Habsburg opposition exhibited by certain magnates, he was crowned in Buda in 1440. The young king set about defending his realm against the Turks, aided by a coalition established by Pope Eugenius IV. In 1443 victories were won in Bulgaria and a useful peace concluded which, in the following year, provoked the Holy See to insist on the preparation of a new expedition aimed at reaching Constantinople. But the Venetian fleet failed to prevent a Turkish attack coming across the Bosphorus, and in 1444 the twenty-year-old Wladyslaw III, along with many Polish and Hungarian knights and the papal legate, were killed at the battle of Varna. This defeat sealed the fate of the Byzantine Empire and the Balkan Slavs. The Ottoman threat now moved ever closer to central Europe. However, neither in Poland nor in Lithuania, where domestic problems predominated, was the importance of the role played by Wladyslaw in the resistance against the Turks fully appreciated.