<P> Constantinople had been an imperial capital since its consecration in 330 under Roman Emperor, Constantine the Great . In the following eleven centuries, the city had been besieged many times but was captured only once: during the Fourth Crusade in 1204 . The crusaders established an unstable Latin state in and around Constantinople while the remaining empire splintered into a number of Byzantine successor states, notably Nicaea, Epirus and Trebizond . They fought as allies against the Latin establishments, but also fought among themselves for the Byzantine throne . </P> <P> The Nicaeans eventually reconquered Constantinople from the Latins in 1261 . Thereafter there was little peace for the much - weakened empire as it fended off successive attacks by the Latins, the Serbians, the Bulgarians, and, most importantly, the Ottoman Turks . The Black Plague between 1346 and 1349 killed almost half of the inhabitants of Constantinople . The city was severely depopulated due to the general economic and territorial decline of the empire, and by 1453 consisted of a series of walled villages separated by vast fields encircled by the fifth - century Theodosian walls . </P> <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, he was just nineteen years old . Many European courts assumed that the young Ottoman ruler would not seriously challenge Christian hegemony in the Balkans and the Aegean . This calculation was boosted by Mehmed's friendly overtures to the European envoys at his new court . But Mehmed's mild words were not matched by actions . By early 1452, work began on the construction of a second fortress (Rumeli hisarı) on the Bosphorus, on the European side several miles north of Constantinople, set directly across the strait on the Asian side from the Anadolu Hisarı fortress, built by his great - grandfather Bayezid I . This pair of fortresses ensured complete control of sea traffic on the Bosphorus; and defended against attack by the Genoese colonies on the Black Sea coast to the north . (This new fortress, was called Boğazkesen, which means' strait - blocker' or' throat - cutter', to emphasize its strategic position .) In October 1452, Mehmed ordered Turakhan Beg to station a large garrison force in the Peloponnese to block Thomas and Demetrios (despotes in Southern Greece) from providing aid to their brother Constantine XI Palaiologos during the impending siege of Constantinople . Michael Critobulus says about the speech of Mehmed II to his soldiers: "My friends and men of my empire! You all know very well that our forefathers secured this kingdom that we now hold at the cost of many struggles and very great dangers and that, having passed it along in succession from their fathers, from father to son, they handed it down to me . For some of the oldest of you were sharers in many of the exploits carried through by them--those at least of you who are of maturer years--and the younger of you have heard of these deeds from your fathers . They are not such very ancient events nor of such a sort as to be forgotten through the lapse of time . Still the eyewitness of those who have seen testifies better than does the hearing of deeds that happened but yesterday or the day before ." 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>

Why was the invasion of the byzantine empire significant