First Battle of the Masurian Lakes
1914 CE.
Shown on the map at 54.000°, 22.000°.
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. By the conclusion of the battle, the Imperial German Army had destroyed the 2nd 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 manpower, and after a couple of weeks the Russians launched a new offensive in East Prussia.
Text from the Wikipedia article First Battle of the Masurian Lakes, licensed CC BY-SA 4.0.