First Battle of the Masurian Lakes

The First Battle of the Masurian Lakes was a German offensive in the Eastern Front 2–16 September 1914, during the second month of World War I. It took place only days after the Battle of Tannenberg where the German Eighth Army encircled and destroyed the Russian Second Army. Using the rapid movements aided by the East Prussian railway network, the Eighth Army reformed in front of the spread-out Russian First Army and pushed them back across their entire front, eventually ejecting it from Germany. Further progress was hampered by the arrival of the Russian Tenth Army on the Germans' right flank.

By the conclusion of the battle, the Imperial German Army had destroyed the Second Army and shattered the First in a series of actions over only a few weeks. However, Russia had the largest army in the world, so the Russian army very quickly restored its losses in soldiers, and after a couple of weeks the Russians launched a new offensive in East Prussia.

Source: Wikipedia