In late spring of 1452 Byzantine Emperor Constantine XI wrote to Pope Nicholas for help against the impending siege of Constantinople by Ottoman Sultan Mehmed the Conqueror. Nicholas issued the bull "Dum Diversas" (18 June 1452) authorizing King Alfonso V of Portugal to "attack, conquer, and subjugate Saracens, pagans and other enemies of Christ wherever they may be found". Issued less than a year before the Fall of Constantinople in 1453, the bull may have been intended to begin another crusade against the Ottoman Empire. Furthermore, the bull "Romanus Pontifex" (1455) gave the right of taking for reason of punishment for crime saracens (who, as Muslims in general were slavers themselves, often capturing Christians) and pagans as perpetual slaves.