The siege of Belgrade, Battle of Belgrade or siege of Nndorfehrvr (Hungarian: Nndorfehrvr ostroma or nndorfehrvri diadal, lit. "Triumph of Nndorfehrvr"; Serbian Cyrillic: , romanized: Opsada Beograda) was a military blockade of Belgrade that occurred July 422, 1456. After the fall of Constantinople in 1453, the Ottoman Sultan Mehmed the Conqueror rallied his resources to subjugate the Kingdom of Hungary. His immediate objective was the border fort of the town of Belgrade (Hungarian: Nndorfehrvr). John Hunyadi, the Count of Temes and captain-general of Hungary, who had fought many battles against the Turks in the previous two decades, prepared the defenses of the fortress.

The siege escalated into a major battle, during which Hunyadi led a sudden counterattack that overran the Ottoman camp, ultimately compelling the wounded Mehmed II to lift the siege and retreat. The battle had significant consequences, as it stabilized the southern frontiers of the Kingdom of Hungary for more than half a century and thus considerably delayed the Ottoman advance in Europe.

As he had previously ordered all Catholic kingdoms to pray for the victory of the defenders of Belgrade, the Pope celebrated the victory by making an enactment to commemorate the day. 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. The day of the victory, July 22, has been a memorial day in Hungary ever since.

A series of military conflicts between the Ottoman Empire and various European states took place from the Late Middle Ages up through the early 20th century. The earliest conflicts began during the Byzantine–Ottoman wars, waged in Anatolia in the late 13th century before entering Europe in the mid 14th century with the Bulgarian–Ottoman wars. In the mid 15th century, the Serbian–Ottoman wars and the Albanian-Turkish wars were waged by Serbia and Albania respectively against the Ottoman Turks. Much of this period was characterized by Ottoman expansion into the Balkans. The Ottoman Empire made further inroads into Central Europe in the 15th and 16th centuries, culminating in the peak of Ottoman territorial claims in Europe.The Ottoman–Venetian Wars spanned four centuries, starting in 1423 and lasting until 1718. This period witnessed the fall of Negroponte in 1470, the fall of Famagusta (Cyprus) in 1571, the defeat of the Ottoman fleet at the Battle of Lepanto in 1571 (at that time the largest naval battle in history), the fall of Candia (Crete) in 1669, the Venetian reconquest of Morea (Peloponnese) in the 1680s and its loss again in 1715. The island of Corfu under Venetian rule remained the only Greek island not conquered by the Ottomans.In the late seventeenth century, European powers began to consolidate against the Ottomans and formed the Holy League, reversing a number of Ottoman land gains during the Great Turkish War of 1683–99. Nevertheless, Ottoman armies were able to hold their own against their European rivals until the second half of the eighteenth century.In the nineteenth century the Ottomans were confronted with insurrection from their Serbian (1804–1817) and Greek (1821–1832) subjects. This occurred in tandem with the Russo-Turkish wars, which further destabilized the empire. The final retreat of Ottoman rule came with the First Balkan War (1912–1913), followed by the signing of the Treaty of Sèvres at the close of World War I.