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.
The union was agreed by the Byzantine Emperor Michael VIII Palaiologos in 1274, at the Second Council of Lyon, and indeed, some Palaiologoi emperors 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.
The imperial efforts to impose union were met with strong resistance in Constantinople.
A propaganda initiative was stimulated 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.
Ultimately, the attempted union between east and west failed, greatly annoying Pope Nicholas V and the hierarchy of the Roman church.

In the summer of 1452, when Rumelı Hisari was completed and the threat of the Ottomans 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.
Furthermore, these Western rulers did not have the wherewithal to contribute to the effort, especially in light of the weakened state of France and England from the Hundred Years' War, Spain's involvement in the Reconquista, the internecine fighting in the Holy Roman Empire, and Hungary and Poland's defeat at the Battle of Varna of 1444.
Although some troops did arrive from the mercantile city-states in northern 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.
An accomplished soldier from Genoa, Giovanni Giustiniani, arrived in January 1453 with 400 men from Genoa and 300 men from Genoese Chios.
As a specialist in defending walled cities, Giustiniani was immediately given the overall command of the defence 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.

Meanwhile, in Venice, deliberations were taking place concerning the kind of assistance the Republic would lend to Constantinople.
The Senate decided upon sending a fleet in February 1453, but the fleet's departure was delayed until April, when it was already too late for ships to assist in battle.
Further undermining Byzantine morale, seven Italian ships with around 700 men, despite having sworn to defend Constantinople, slipped out of the capital the moment when Giustiniani arrived.
At the same time, Constantine's attempts to appease the Sultan with gifts ended with the execution of the Emperor's ambassadors.

Fearing a possible naval attack along the shores of the Golden Horn, Emperor Constantine XI ordered that a defensive chain be placed at the mouth of the harbour.
This chain, which floated on logs, was strong enough to prevent any Turkish ship from entering the harbour.
This device was one of two that gave the Byzantines some hope of extending the siege until the possible arrival of foreign help.
This strategy was enforced because in 1204, the armies of the Fourth Cr
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. The union was agreed by the Byzantine Emperor Michael VIII Palaiologos in 1274, at the Second Council of Lyon, and indeed, some Palaiologoi emperors 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. The imperial efforts to impose union were met with strong resistance in Constantinople. A propaganda initiative was stimulated 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. Ultimately, the attempted union between east and west failed, greatly annoying Pope Nicholas V and the hierarchy of the Roman church.
In the summer of 1452, when Rumelı Hisari was completed and the threat of the Ottomans 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. Furthermore, these Western rulers did not have the wherewithal to contribute to the effort, especially in light of the weakened state of France and England from the Hundred Years' War, Spain's involvement in the Reconquista, the internecine fighting in the Holy Roman Empire, and Hungary and Poland's defeat at the Battle of Varna of 1444. Although some troops did arrive from the mercantile city-states in northern 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. An accomplished soldier from Genoa, Giovanni Giustiniani, arrived in January 1453 with 400 men from Genoa and 300 men from Genoese Chios. As a specialist in defending walled cities, Giustiniani was immediately given the overall command of the defence 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.
Meanwhile, in Venice, deliberations were taking place concerning the kind of assistance the Republic would lend to Constantinople. The Senate decided upon sending a fleet in February 1453, but the fleet's departure was delayed until April, when it was already too late for ships to assist in battle. Further undermining Byzantine morale, seven Italian ships with around 700 men, despite having sworn to defend Constantinople, slipped out of the capital the moment when Giustiniani arrived. At the same time, Constantine's attempts to appease the Sultan with gifts ended with the execution of the Emperor's ambassadors.
Fearing a possible naval attack along the shores of the Golden Horn, Emperor Constantine XI ordered that a defensive chain be placed at the mouth of the harbour. This chain, which floated on logs, was strong enough to prevent any Turkish ship from entering the harbour. This device was one of two that gave the Byzantines some hope of extending the siege until the possible arrival of foreign help. This strategy was enforced because in 1204, the armies of the Fourth Cr
Fall of Constantinople