<P> By 1450 the empire was exhausted and had shrunk to a few square miles outside the city of Constantinople itself, the Princes' Islands in the Sea of Marmara, and the Peloponnese with its cultural center at Mystras . The Empire of Trebizond, an independent successor state that formed in the aftermath of the Fourth Crusade, also survived on the coast of the Black Sea . </P> <P> When Sultan Mehmed II succeeded his father in 1451, it was widely believed that the young ruler, then 19 years old, would prove incapable--and that he would pose no great threat to Christian possessions in the Balkans and the Aegean . This optimism was reinforced by friendly assurances made by Mehmed to envoys sent to his new court . But Mehmed's actions spoke far louder than his mild words . Beginning early in 1452, he built a second Ottoman fortress on the Bosphorus, on the European side several miles north of Constantinople, set directly across the strait from the similar fortress, Anadolu Hisarı, which his great - grandfather Bayezid I had previously built on the Asian side . This pair of fortresses gave the Turks complete control of sea traffic on the Bosphorus; specifically, it prevented help from the north, the Genoese colonies on the Black Sea coast, from reaching Constantinople . (The new fortress was also known as Boğazkesen, which held the dual meanings' strait - blocker' or' throat - cutter', emphasizing its strategic position .) In October 1452, Mehmed ordered Turakhan Beg to lead a large force into the Peloponnese and remain there to keep Thomas and Demetrios from assisting their brother Constantine XI Palaiologos during the impending siege of Constantinople . </P> <P> Byzantine Emperor Constantine XI swiftly understood Mehmed's true intentions and turned to Western Europe for help; but now the price of centuries of war and enmity between the eastern and western churches had to be paid . Since the mutual excommunications of 1054, the Pope in Rome was committed to establishing authority over the eastern church . Nominal union had been negotiated in 1274, at the Second Council of Lyon, and indeed, some Palaiologoi emperors (Latin, Palaeologan) had since been received into the Latin church . Emperor John VIII Palaiologos had also recently negotiated union with Pope Eugene IV, with the Council of Florence of 1439 proclaiming a Bull of Union . These events, however, stimulated a propaganda initiative by anti-unionist Orthodox partisans in Constantinople; the population, as well as the laity and leadership of the Byzantine Church, became bitterly divided . Latent ethnic hatred between Greeks and Italians, stemming from the events of the Massacre of the Latins in 1182 by the Greeks and the sack of Constantinople in 1204 by the Latins, played a significant role . Finally, the attempted Union failed, greatly annoying Pope Nicholas V and the hierarchy of the Roman church . </P> <P> In the summer of 1452, when Rumelı Hisari was completed and the threat had become imminent, Constantine wrote to the Pope, promising to implement the Union, which was declared valid by a half - hearted imperial court on 12 December 1452 . Although he was eager for an advantage, Pope Nicholas V did not have the influence the Byzantines thought he had over the Western kings and princes, some of whom were wary of increasing Papal control, and these had not the wherewithal to contribute to the effort, especially in light of the weakened state of France and England from the Hundred Years' War, Spain being in the final part of the Reconquista, the internecine fighting in the German Principalities, and Hungary and Poland's defeat at the Battle of Varna of 1444 . Although some troops did arrive from the mercantile city states in the north of Italy, the Western contribution was not adequate to counterbalance Ottoman strength . Some Western individuals, however, came to help defend the city on their own account . Cardinal Isidore, funded by the pope, arrived in 1452 with 200 archers One of these was an accomplished soldier from Genoa, Giovanni Giustiniani, who arrived with 400 men from Genoa and 300 men from Genoese Chios, in January 1453 . As a specialist in defending walled cities, he was immediately given the overall command of the defense of the land walls by the emperor . Around the same time, the captains of the Venetian ships that happened to be present in the Golden Horn offered their services to the Emperor, barring contrary orders from Venice, and Pope Nicholas undertook to send three ships laden with provisions, which set sail near the end of March . </P>

What geographic advantage did constantinople have over rome