This article is about the 1456 siege of Nándorfehérvár or Belgrade, but the fort was also besieged in 1718 and in 1788.

After the fall of Constantinople in 1453, Ottoman sultan Mehmed II was rallying his resources in order to subjugate Hungary. His immediate objective was Belgrade, which at the time was a Hungarian végvár (border fort) of Nándorfehérvár, today's Belgrade. János (John) Hunyadi, a Hungarian nobleman and warlord from a Wallachian (Romanian) lineage who fought many battles against the Ottomans in the previous two decades, expected just such an attack.

At the end of 1455, after a public reconciliation with all his enemies, he began preparations. At his own expense he provisioned and armed the fortress, and leaving in it a strong garrison under the command of his brother-in-law Mihály Szilágyi and his own eldest son László, he proceeded to form a relief army and a fleet of two hundred corvettes. As no other baron was willing to help (fearing Hunyadi's gorwing power more than the Ottoman threat), he was left entirely to his own resources.

His one ally was the Franciscan friar, Giovanni da Capistrano, who preached a crusade so effectually that the peasants and yeomanry, ill-armed (most of them had but slings and scythes) but full of enthusiasm, flocked to the standard of Hunyadi, the kernel of whose host consisted of a small band of seasoned mercenaries and a few banderia of noble horsemen. All in all, Hunyadi could build a force of 25-30,000 men.

However, before these forces could have been assembled, Mehmet II's invasion army (160,000 men in early accounts, 60-70,000 according to newer research) arrived at Nándorfehérvár. On July 4, 1456, the siege began. Szilágyi could only rely on a force of 5-7,000 men in the castle.

On the 14th of July 1456 Hunyadi arrived with his flotilla on the Danube, and destroyed the Sultan's fleet. The fort's defense could be reinforced. But after a week of heavy bombardement, on the 21st the besieging army flooded the city, and then started its assault on the fort. But Szilágyi's soldiers beat off a fierce assault. When a Turkish soldier almost managed to pin the Sultan's flag on top of a bastion, a soldier named Titus Dugovic (Dugovics Titusz in Hungarian) grabbed him and together they plunged from the wall. (For this heroism John Hunyadi's son, Hungarian king Matthias Corvinus made Titus's son a nobleman three years later.)

In the second day of the onslaught, Capistrano led his crusaders to the Turkish rear army across the Sava river. At the same time, Hunyadi started a desperate charge out of the fort to take the cannon positions in the Turkish camp. (By some accounts, the peasant crusaders started their action spontaneously on their own, and Capistrano and Hunyadi only made use of the situation). The surprise attacks caused heavy losses and much disarray, thus during the night Mehmed withdrew his remaining forces and returned to Constantinople (now Istambul). The Magyars had, however, to pay dearly for this victory, as plague broke out in the camp, in which John Hunyadi himself died three weeks later (11th August 1456).

The victory stopped the Ottoman Turkish advance towards Catholic Europe for 70 years, though they made other incursions such as the taking of Otranto in 1480-1481 and the raid of Croatia and Styria in 1493.

During the siege, Pope Callixtus III ordered the noon bell , to call believers to pray for the defenders - but as in many places the news of victory arrived later than the order, it transformed into the commemoration of the victory, and the Pope didn't withdraw the order. Hence the noon bell is still rung to this day.