"Triumph of Nándorfehérvár"; Serbian Cyrillic: Опсада Београда, romanized: Opsada Beograda) was a military blockade of Belgrade that occurred 4–22 July 1456 in the aftermath of the fall of Constantinople in 1453 marking the Ottomans' attempts to expand further into Europe.

The Hungarian defenders, under the leadership of John Hunyadi, who had garrisoned and strengthened the fortress city at his own expense, put up a determined resistance against the larger Ottoman army.

This victory boosted the morale of European Christian forces and was seen as a turning point in their efforts as it provided a crucial buffer and temporarily halted Ottoman expansion in Europe.

John Hunyadi's successful defence of Belgrade earned him widespread acclaim and respect as a military leader though he died of the plague a few weeks later.

The battle's significance also extended beyond its immediate aftermath, as it demonstrated the importance of firearms and artillery in warfare, heralding a new era in military technology and tactics.

They considered Belgrade, a strategically positioned fortress city at the confluence of the Danube and Sava river, as a crucial gateway for their advance northward.

[13] After conducting two campaigns against the Serbian Despotate in 1454 and 1455, Sultan Mehmed decided to continue his conquests towards the northwest by capturing the strategically important city of Belgrade from the Kingdom of Hungary.

Significant preparations were made by the Ottoman sultan for the conquest of the city, including the casting of 22 large cannons alongside many smaller ones and the establishment of a navy that would sail up the Danube to assist the army during the siege.

[16] An Italian Franciscan friar allied to Hunyadi, Giovanni da Capistrano, sent as an inquisitor to Hungary to eliminate or convert so-called heretics, (non-Catholics)[17] started preaching a crusade to attract peasants and local countryside landlords to join the defence of Europe.

[19] Before Hunyadi could assemble his forces, the Ottoman troops from the army of Mehmed II (160,000 men in early accounts, 60–70,000 according to newer research) started appearing near Belgrade in the last days of June.

Capistrano, the Friar sent to Hungary by the Vatican both to find heretics and to preach a crusade against the Ottomans, managed to raise a large, albeit poorly trained and equipped, peasant army, with which he advanced towards Belgrade.

[14] Instead, the decision was made to prioritize capturing the city from its besieged defenders, a move seen as a tactical blunder by modern historians, as it allowed Hunyadi to set up camp across the river uncontested.

This building endeavour was one of the most elaborate military architecture achievements of the Middle Ages as it also benefitted from the natural obstacle of the rivers being at the junction of the Danube and the Sava.

[22] Hunyadi established his camp in the vicinity of the Zemun fortress, while the Ottoman fleet encircled Belgrade along the Danube to put a stop to the provisioning of the city.

To achieve this, he commanded the assembly of all ships on the Danube and communicated with Szilágyi, instructing him to be prepared to launch an attack on the Ottoman fleet from a strategic position.

The battle between the encircled Janissaries and Szilágyi's soldiers inside the upper town was turning in favour of the Christians, and the Hungarians managed to beat off the fierce assault from outside the walls.

The Janissaries remaining inside the city were thus massacred while the Ottoman troops trying to breach the upper town suffered heavy losses.

[24][26] At this crucial point of the battle, one of the viziers advised Mehmed to abandon the camp for his safety, which he refused to do so on the grounds that it would be a “sign of cowardice”.

[14] During the night of July 22 to 23, the Ottomans buried their dead according to their customs, loaded their wounded in a long row of wagons, and evacuated the camp in a hurry, heading southeast.

[24] It is claimed that after the defeat, while he and his army were retreating into Bulgaria, the failure as well as the ensuing loss of no less than 24,000 of his best soldiers angered Mehmed in such a manner that, in an uncontrollable fit of fury, he wounded a number of his generals with his own sword, before ordering their executions.

Pope Callixtus III ordered the bells of every European church to be rung every day at noon, as a call for believers to pray for the defenders of the city.

This led to the legend that the noon bell ritual undertaken in Catholic and old Protestant churches, enacted by the Pope before the battle, was founded to commemorate the victory.

[37] The victory stopped the Ottoman advance towards Europe beyond the Balkans for 70 years, though they made other incursions such as the taking of Otranto between 1480 and 1481; and the raid of Croatia and Styria in 1493.

Wallachia, the Crimean Khanate, and eventually Moldavia were merely converted into vassal states due to the strong military resistance to Mehmed's attempts of conquest.