On May 3, 1915, during a 10-day-long stretch of fighting in the Carpathian Mountains on the Galician front in Austria-Hungary, a combined Austro-German force succeeds in defeating the Russian army near the Dunajec River (a tributary of the Vistula River that runs through modern-day northern Slovakia and southern Poland).
The Austro-German counterattack in Galicia in early May 1915 decisively ended nine months of victorious Russian advances in the region since August 1914. Struggling, Austria-Hungary had appealed to its more powerful ally, and the German army had stepped in, moving large amounts of troops into the region in an attempt to break through the Russian lines between the crest of the Carpathians and the mid-section of the Dunajec. On May 1, 1915, the German commander General August von Mackensen led the combined troops into battle behind an artillery bombardment by 610 guns, the largest yet on the Eastern Front, against Russian positions stretching along a 40-kilometer front.
Within 24 hours, the Russians had been driven out of the city of Gorlice, in western Galicia (modern-day Poland). Mackensen reported of the attacks that: The enemy had been so shaken by the heavy artillery fire that his resistance at many points was very slight. In headlong flight he left his defenses, when the infantry of the [Teutonic] allies appeared before his trenches, throwing away rifles and cooking utensils and leaving immense quantities of infantry ammunition and dead. Mackensen's forces continued their rapid progress over the next several days, crossing the Dunajec, 65 kilometers north of the city of Krakow, by May 3.
By May 6, the Russians had also abandoned the city of Tarnow, to the north of Gorlice, suffering heavy casualties; thousands lay dead on the battlefield, and the prisoners taken during the 10 days of battle would eventually number 143,500. As a result of the Austro-German offensive, the Russians were forced to abandon their positions in the Carpathians and withdraw many of their forces in the region to the vicinity of the fortress of Przemysl, the former location of Austrian headquarters until its fall to the Russians the previous March.
As Stanley Washburn, a British military observer assigned to the Russian forces, wrote of the Gorlice-Tarnow offensive, the battle was lopsided from the beginning: Russia is not an industrial nation, and cannot turn her resources into war material overnight as the Germans have been able to do. She was outclassed in everything except bravery, and neither the Germans nor any other army can claim superiority to her in that respect. With the centre literally cut away, the keystone of the Russian line had been pulled out, and nothing remained but to retire.