Prussia's reversal in Bohemia paralleled the entry of new belligerents on the Austrian side. In mid-1757 a Russian force of 75,000 troops under Field Marshal Stepan Fyodorovich Apraksin invaded East Prussia and took the fortress at Memel. Advancing further, the Russians engaged and defeated a smaller Prussian force led by Lehwaldt in the Battle of Gross-Jägersdorf on 30 August. However, the victorious Russians were unable to take Königsberg, having expended their supplies at Memel and Gross-Jägersdorf, and retreated soon afterwards; recurring difficulties with logistics limited the offensive capabilities of the large Russian army and allowed East Prussia to hold out longer than might have been expected. Sweden, too, declared war on Prussia in September, invading Prussian Pomerania on 13 September with 17,000 men and beginning the Pomeranian War. The need to defend core territories on these fronts reduced Prussia's offensive capacity in Bohemia and Silesia.