Seven Years%27 War
Later that summer, the Russians invaded Memel with 75,000 troops. Memel had one of the strongest fortresses in Prussia. However, after five days of artillery bombardment the Russian army was able to storm it. The Russians then used Memel as a base to invade East Prussia and defeated a smaller Prussian force in the fiercely contested Battle of Gross-Jägersdorf on 30 August 1757. However, it was not yet able to take Königsberg and retreated soon afterward. Still, it was a new threat to Prussia. Not only was Frederick forced to break off his invasion of Bohemia, he was now forced to withdraw further into Prussian-controlled territory. His defeats on the battlefield brought still more opportunist nations into the war. Sweden declared war on Prussia and invaded Pomerania with 17,000 men. Sweden felt this small army was all that was needed to occupy Pomerania and felt the Swedish army would not need to engage with the Prussians because the Prussians were occupied on so many other fronts.

Q: What caused the Russians based in Memel to be successful deeper into Prussia?
defeated a smaller Prussian force