In 1428, while the Ottoman Empire was fighting a war with the Republic of Venice, the Ottomans and the Kingdom of Hungary achieved a temporary peace by establishing the Serbian Despotate as a buffer state. After the war ended in 1430, the Ottomans returned to their earlier policy of controlling all lands south of the Danube. In 1432, Sultan Murad II began raiding into Transylvania. After King Sigismund died in 1437, the attacks intensified, with the Ottomans occupying Borač in 1438 and Zvornik and Srebrenica in 1439. At the end of 1439, Smederevo capitulated and Murad succeeded in making Serbia an Ottoman province. Đurađ Branković, Despot of Serbia, fled to his estates in Hungary. In 1440, Murad besieged Hungary's main border fortress, Belgrade. After failing to take the fortress, he was f