Second Battle of the Masurian Lakes
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The Second Battle of the Masurian Lakes, also known as the Winter Battle of the Masurian Lakes, was the northern part of the Central Powers' offensive on the Eastern Front in the winter of 1915. The offensive was intended to advance beyond the Vistula River and perhaps knock Russia out of the war.
The Central Powers planned four offensives on their Eastern Front in early 1915. The Germans, led by Paul von Hindenburg, would attack eastward from their front line in western Poland, which had been occupied after the Battle of Łódź in 1914, toward the Vistula River and also in East Prussia in the vicinity of the Masurian Lakes (site of the 1914 Battle of the Masurian Lakes). The Austro-Hungarians would emerge from the Carpathian Mountain passes to attack the Russians by driving toward Lemberg. They would be led by General Alexander von Linsingen. Further south General Borojevic von Bojna would attempt to relieve the besieged fortress at Przemysl.
German Chief of Staff Erich von Falkenhayn strongly believed that the war would be won on the Western Front. Nonetheless he sent four additional army corps to Paul von Hindenburg, commander of their Eastern Front.  By February 1915 thirty six percent of the German field army was in the east. 
German Ninth Army attacked from Silesia into Poland at the end of January; they released tear gas, which stopped their assault by blowing back on the attackers. The Russians counterattacked with eleven divisions under a single corps commander, losing 40,000 men in three days. In East Prussia further Russian incursions were blocked by trench lines extending between the Masurian Lakes; they were held by German Eighth Army, commanded by General Otto von Below. It was reinforced by some of the newly arrived corps, while the rest of them became the German Tenth Army, commanded by Colonel-General Hermann von Eichhorn, which was formed on the German left. It was to be one wing of a pincers intended to surround their opponents: General Sievers' Russian Tenth Army. A new Russian Twelfth Army under General Pavel Plehve was assembling in Poland roughly 100 km (62 mi) to the southwest. 
Sievers warned the Northwest Front commander, General Nikolai Ruzsky, that they were likely to be attacked, but was ignored. On February 7, despite a heavy snowstorm, the left wing of Below's Eighth Army launched a surprise attack against Sievers, whose trenches were shallow, disconnected ditches, with little or no barbed wire because the first shipments had not arrived until December 1914.  The following day the German Tenth Army also drove forward. Snow, with drifts as high as a man, slowed German progress down the roads for the first two days; off the roads the ground was too boggy for fighting. Despite these formidable obstacles the Germans pincers advanced 120 km (75 mi) in a week, inflicting severe casualties on the Russians. As the Russians withdrew the center of the German Eight Army also began to thrust forward. The Russian withdrawal was disorderly so many prisoners were taken. Russian counterattacks on the lengthening flank of German Tenth Army were beaten back. German men and horses fed on captured provisions, so only ammunition had to be hauled up to them. Then the snow was washed away by torrential rain. The climax of the battle was on 18 February when Russian 20th Army Corps, under General Bulgakov, was surrounded by the German Tenth Army in the vast Augustow Forest. On February 21 the survivors from the corps surrendered.
The heroic stand of the Russian 20th Corps provided the time required for the rest of the Russian Tenth Army to form a new defensive position. On February 22, the day after 20th Corps surrender, Plehve's Russian Twelfth Army counterattacked, which checked further German advances and brought the battle to an end. One source gives Russian losses as 92,000 prisoners and 300 guns  while another gives 56,000 men and 185 guns.  The Germans lost 7,500 men and 14 guns  The Germans besieged the Russian fortress at Osowiec, but were unable to take it. 
The Second Battle of the Masurian Lakes gave the Germans a toehold in Russia, however the Russians blocked further advances. In the following weeks the Germans drove the Russians out of their remaining small enclaves in East Prussia. 
Further south, Alexander von Linsingen's offensive had failed with severe losses, and the fortress at Przemysl had been forced to surrender to the Russians. Clearly the first Austro-Hungarian offensives of 1915 were abject failures. Henceforth the Austro-Hungarians and Germans would work closer together (see the Gorlice–Tarnów Offensive
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- The Second Battle of the Masurian Lakes, 1915