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Conquest of Serbia (1454–1459)

Mehmed II's first campaigns after Constantinople were in the direction of Serbia, which had been an Ottoman vassal state intermittently since the Battle of Kosovo in 1389. The Ottoman ruler had a connection with the Serbian Despotate – one of Murad II 's wives was Mara Branković – and he used that fact to claim Serbian lands. Đurađ Branković 's recently made alliance with the Hungarians, and his irregular payments of tribute, further served as justifications for the invasion. The Ottomans sent an ultimatum demanding the keys to some Serbian castles which formerly belonged to the Ottomans. When Serbia refused these demands, the Ottoman army led by Mehmed set out from Edirne towards Serbia in 1454, sometime after the 18th of April. Mehmed's forces quickly succeeded in capturing Sivricehisar (sometimes identified with the Ostrvica Fortress) and Omolhisar, and repulsed a Serbian cavalry force of 9,000 cavalry sent against them by the despot. Following these actions, the Serbian capital of Smederevo was put under siege by the Ottoman forces. Before the city could be taken, intelligence was received about an approaching Hungarian relief force led by Hunyadi, which caused Mehmed to lift the siege and start marching back to his domains. By August the campaign was effectively over, Mehmed left a part of his force under the command of Firuz Bey in Serbia in anticipation of a possible offensive on Ottoman territories by Hunyadi. This force was defeated by a combined Hungarian-Serbian army led by Hunyadi and Nikola Skobaljić on the 2nd of October near Kruševac, after which Hunyadi went on to raid Ottoman controlled Nish and Pirot before returning back to Belgrade. Roughly a month later, on the 16th of November, the Ottomans avenged their earlier defeat at Kruševac by defeating Skobaljić's army near Tripolje, where the Serbian voivode was captured and executed via impalement. Following this a temporary treaty was signed with the Serbian despot, where Đurađ would formally recognize the recently captured Serbian forts as Ottoman land, send thirty thousand florins to the Porte as yearly tribute and provide troops for Ottoman campaigns. The 1454 campaign had resulted in the capture of fifty thousand prisoners from Serbia, four thousand of whom were settled in various villages near Constantinople. The following year, Mehmed received reports from one of his frontier commanders about Serbian weakness against a possible invasion, the reports in combination with the dissatisfactory results of the 1454 campaign convinced Mehmed to initiate another campaign against Serbia. The Ottoman army marched on the important mining town of Novo Brdo, which Mehmed put under siege. The Serbians couldn't resist the Ottoman army out in the open, thus resorted to fortifying their various settlements and having their peasants flee to either various fortresses or forests. After forty days of siege and intense cannon fire, Novo Brdo surrendered. Following the conquest of the city, Mehmed captured various other Serbian settlements in the surrounding area, after which he started his march back towards Edirne, visiting his ancestor Murad I 's grave in Kosovo on the way.

In 1456, Mehmed decided to continue his momentum towards the Northwest and capture the previously Serbian controlled city of Belgrade, now ruled by the Kingdom of Hungary. The Ottoman army advanced to walls of Belgrade, where it attempted but failed to conquer the city from John Hunyadi at the Second Ottoman Siege of Belgrade, on 14 July 1456. A period of relative peace ensued in the region until the Fall of Belgrade in 1521, during the reign of Mehmed's great-grandson, known as Sultan Suleiman the Magnificent. The sultan retreated to Edirne, and Đurađ Branković regained possession of some parts of Serbia. Before the end of the year, however, the 79-year-old Branković died. Serbian independence survived him for only two years, when the Ottoman Empire formally annexed his lands following dissension among his widow and three remaining sons. Lazar, the youngest, poisoned his mother and exiled his brothers, but he died soon afterwards. In the continuing turmoil the oldest brother Stefan Branković gained the throne but was ousted in March 1459. After that the Serbian throne was offered to Stephen Tomašević, the future king of Bosnia, which infuriated Sultan Mehmed. He sent his army, which captured Smederevo in June 1459, ending the existence of the Serbian Despotate.