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 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.

The Eastern Front or Eastern Theater of World War I was a theater of operations that encompassed at its greatest extent the entire frontier between Russia and Romania on one side and Austria-Hungary, Bulgaria, the Ottoman Empire, and Germany on the other. It ranged from the Baltic Sea in the north to the Black Sea in the south, involved most of Eastern Europe, and stretched deep into Central Europe. The term contrasts with the Western Front, which was being fought in Belgium and France.