After the fall of Constantinople in 1453, the Ottoman Sultan Mehmed the Conqueror rallied his resources to subjugate the Kingdom of Hungary. Without formally declaring an end to the peace treaty, Sultan Mehmed II invaded Serbia in mid-July 1454. His immediate objective was the border fort of the town of Belgrade. Much of central Serbia fell, but the capital was well-prepared and the Ottomans, upon hearing that Hunyadi would cross the Danube to reinforce the Serbs, soon lifted their siege of Smederevo. The Sultan retreated back to Sofia with loot and slaves, leaving most of his army at Kruševac.