The Conquest of Poland — Gorlice-Tarnów
Having checked Russia’s offensive in 1914 into East Prussia at the battles of Tannenberg (Stębark) and Masurian (Mazury) lakes, Germany went on the offensive on the eastern front in 1915, occupying Russian-controlled Poland and pushing deep into Russia. In February 1915, in the second battle of the Masurian lakes, the Germans pushed the badly deployed Russian Tenth Army back 100 kilometres (60 miles) to Kovno (Kaunas), clearing East Prussia of Russian forces. But Russian forces to the south still occupied Poland and stood at the mouth of the Carpathian passes, threatening Austria-Hungary. In March 1915, the Russians finally took the besieged Austro-Hungarian fortress of Przemyśl, along with 120,000 Austro-Hungarian prisoners. The Russian menace to Austria-Hungary compelled the Germans to act to ensure the survival of their ally, more especially as Italy joined the war on 23 May against Austria-Hungary.