Mehmed II's first campaigns after Constantinople were in the direction of Serbia. Mehmed led a campaign against Serbia because the Serbian ruler Đurađ Branković refused to send tribute and made an alliance with the Kingdom of Hungary. The Ottoman army conquered the important mining city of Novo Brdo. The Ottoman army advanced as far as Belgrade, where it attempted but failed to conquer the city from John Hunyadi at the Siege of Belgrade, on 14 July 1456.