The siege of Belgrade (Hungarian: Nándorfehérvár ostroma) in 1521 is an event that followed as a result of the third major Ottoman attack on this Hungarian stronghold in the Ottoman–Hungarian wars at the time of the greatest expansion of the Ottoman Empire to the west. Ottoman Sultan Suleiman the Magnificent launched his army in mid-May 1521. The Hungarian state was almost in disarray and unable to effectively counter the Ottoman army.

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