The First Battle of the Masurian Lakes was a World War I battle fought from September 9 to September 14, 1914, between Germany and Russia. It was the second victory of the Germans over the Russians as they pushed the Russian 1rst Army back across its entire front, upsetting the Russian hopes of a successful invasion of East Prussia.
Having defeated the Russian 2nd Army, commanded by Samsonov, at Tannenberg, Paul von Hindenburg’s 8th Army turned their attention to the Russian First Army, commanded by Rennenkampf. These two Russian armies had been deployed as the two jaws of a pincer movement with the purpose of snapping up Hindenburg‘s forces in East Prussia. With one jaw of the pincers already destroyed, Hindenburg determined to neutralize the other in short order. If Rennenkampf’s force could be taken out, then Germany’s eastern border would be much more secure from an attack and there would be the possibility to transfer men to the Western Front.
The German 8th Army consisted of 21 divisions of which 18 were infantry and 3 cavalry. Upon receiving news of the 2nd Army’s defeat, Rennenkampf ordered his forces to retreat to a firmer position extending from the Baltic south-east to Angerburg. The Germans started their attack from both sides of the southern lakes on September 7, 1914. Being outnumbered 3-to-1, the some Russian units scattered as the Germans continued to advance northwards in pursuit of the main body of the Russian army.
As he was about to be outflanked, Rennenkampf authorized a further withdrawal on September 9, launching a small-scale counter-strike at the same time with two divisions to hold up the German advance so that his army could pull back orderly. By September 14, Rennenkampf had succeeded in withdrawing his men from Prussia, but at a very high cost, for the Battle of Masurian Lakes of 1914 cost the Russian 1st Army 125,000 men and 150 artillery guns.
Thus, The Germans had removed any threat to East Prussia. As a consequence of Tannenberg and the Masurian Lakes, Hindenburg was hailed as a hero in Germany, subsequently succeeding Falkenhayn as Chief of the German Staff in late summer 1916.