Frederick had previously occupied Dresden in 1756 during his Invasion of Saxony, which had triggered the outbreak of war. In 1759 it had been taken back by Austria-led forces. Frederick now targeted it in an attempt to reassert control over Saxony where he had expansionist territorial ambitions. Dresden was the third major siege Frederick had been forced to abandon following Prague (1757) and Olomouc (1758). Frederick's forces crossed into Silesia and fought the Battle of Liegnitz on 15 August.