The Battle of Masurian Lakes was fought in September 1914. Masurian Lakes was the second defeat for the Russian Army in World War One at the hands of the German army - the previous month had seen the destruction of the Russian Second Army at the Battle of Tannenburg.
Just as Ludendorff and Hindenburg had seen off Samsonov at Tannenburg, they wanted the second part of what had been intended as a Russian pincer movement in East Prussia defeated - Rennenkampf's Russian First Army. With Samsonov's Second Army defeated, Rennenkampf was in a much weaker position in East Prussia. If Rennenkampf's force was taken out, then Germany's eastern border was much more secure from attack and the potential existed to transfer men to the Western Front.
The battle at the Masurian Lakes was not as one-sided as Tannenburg had been. The Germans started their attack on September 7th 1914. By September 9th, Rennenkampf, his army outnumbered by 3 to 1, ordered an orderly withdrawal. Two divisions were left behind to slow down the German advance and to allow the Russians the opportunity to withdraw unhindered.
By September 13th, Rennenkampf had achieved this orderly withdrawal but Russia had pulled all of her troops out of East Prussia. However, Rennenkampf had the semblance of his army left - unlike the Russian Second Army. Despite Rennenkampf's success in withdrawing his men from Prussia, the battle at Masurian Lakes still cost the Russian First Army 125,000 men and 150 artillery guns. The Germans lost 40,000 men.