First Battle of the Masurian Lakes

Following the Tannenberg victory, German forces under Hindenburg and Ludendorff turned against the Russian First Army under Rennenkampf, driving it back through the Masurian Lakes district and across the frontier with 125,000 casualties. East Prussia was cleared of Russian forces by mid-September. The twin German victories at Tannenberg and the Masurian Lakes established the legend of Hindenburg and Ludendorff, and freed Germany to reinforce the Western Front.