 Complete the following phrase:  Constantine XIs reign would prove to be brief. Murad IIs young son and successor, Mehmed II, who became sultan in 1451, was obsessed with the idea of conquering Constantinople. In an effort to extort money from Mehmed, Constantine implicitly threatened to release Orhan Çelebi, Mehmeds cousin and the only other known living member of the Ottoman dynasty and as such a potential rival to Mehed, who was held prisoner in Constantinople. The ill-advised threat gave Mehmed a casus belli and late in 1451, preparations were already underway for a new Ottoman siege of Constantinople. To prevent aid coming from the Morea, now governed by Thomas and Demetrios, Mehmed II sent one of his generals, Turahan Bey who had raided the Morea twice before to devastate the peninsula. Constantine also sent desperate pleas for aid to Western Europe, though little help ever arrived. After a 53-day long siege, the city finally 