Gorlice–Tarnów_Offensive

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.: 201