The First Battle of the Masurian Lakes was a German offensive in the Eastern Front 2–16 September 1914, during the Russian invasion of East Prussia. It took place only days after the Battle of Tannenberg where the German 8th Army encircled and destroyed the Russian 2nd Army. Using the rapid movements aided by the East Prussian railway network, the 8th Army reformed in front of the spread-out Russian 1st Army and pushed it back across its entire front, eventually ejecting it from Germany. Further progress was hampered by the arrival of the Russian 10th Army on the Germans' right flank.