The First Battle of the Masurian Lakes was a German offensive on the Eastern Front during the early part of World War I. It started on 7 September and ended on 14 September 1914. The battle was the second heavy defeat of a Russian army by the Germans. The first defeat was at the Battle of Tannenberg. At Masurian Lakes the Germans pushed the Russian 1st Army back out of Germany. They could not push the Russians back further because of the arrival of the Russian Tenth Army on the Germans' left flank (side). The Russians suffered a defeat and retreated losing about 125,000 men and 150 guns.