The Gorlice–Tarnów offensive during World War I was initially conceived as a minor German offensive to relieve Russian pressure on the Austro-Hungarians to their south on the Eastern Front, but resulted in the Central Powers' chief offensive effort of 1915, causing the total collapse of the Russian lines and their retreat far into Russia. The continued series of actions lasted the majority of the campaigning season for 1915, starting in early May and only ending due to bad weather in October. Mackensen viewed securing a breakthrough as the first phase of an operation, which would then lead to a Russian retreat from the Dukla Pass, and their positions north of the Vistula. Background In the early months of war on the Eastern Front, the German Eighth Army conducted a series of almost miraculous actions against the two Russian armies facing them. After surrounding and then destroying the Russian Second Army at the Battle of Tannenberg in late August, Paul von Hindenburg and Erich Ludendorff wheeled their troops to face the Russian First Army at the First Battle of the Masurian Lakes, almost destroying them before they reached the protection of their own fortresses as they retreated across the border. At the same time, the Austro-Hungarian Army launched a series of attacks collectively known as the Battle of Galicia that were initially successful but soon turned into a retreat that did not stop until reaching the Carpathian Mountains in late September. Over the next weeks, Russian troops continued to press forward into the Carpathian passes in the south of Galicia. In fierce winter fighting General Franz Conrad von Hötzendorf, the chief of staff of the Austro-Hungarian Army, attacked the Russians in an attempt to push them back. Both sides suffered appallingly, but the Russians held their line. By this time half of the Austro-Hungarian Army that had entered the war were casualties. Conrad pleaded for additional German reinforcements to hold the passes. German Chief of German Great General Staff Erich von Falkenhayn refused, but in April 1915 Conrad threatened a separate peace if the Germans would not help. According to Prit Buttar, "...it did seem as if the Russian Army had been gravely weakened by the recent campaigns...both AOK and OHL knew about Russian losses and difficulties with ammunition supply. Therefore, merely reducing the pressure on the k.u.k. Army would not be sufficient; Falkenhayn wished to strike a blow that would