The Gorlice-Tarnow Offensive was a major German and Austro-Hungarian offensive on the Eastern Front of World War I which was launched from 2 May to 22 June 1915. The offensive was a decisive victory for the Central Powers, as it forced the Imperial Russian Army to embark on the humiliating "Great Retreat" into Russia.
In February 1915, the Germans defeated the Imperial Russian Army in East Prussia at the Second Battle of the Masurian Lakes. On the Carpathian front, in March, Austria-Hungary was rocked by the fall of the fortress of Przemysl and its 120,000-strong garrison after a Russian siege lasting 133 days. Neither side made much progress in fighting in the high Carpathian passes. Considering that Russian soldiers had been short of every form of equipment, from rifles, bullets, and shells to boots and overcoats, they had put up a creditable performance on both fronts.
Animosity between German Chief of the General Staff Erich von Falkenhayn and the Hindenburg-Ludendorff partnership shaped the next moves. Rejecting Hindenburg and Ludendorff's plans for an offensive in East Prussia, Falkenhayn concentrated his resources on a new Eleventh Army under General August von Mackensen in northern Galicia. Mackensen was also given effective command of the Austro-Hungarian 4th Army.
On 2 May, this Austro-German force launched an offensive between Gorlice and Tarnow, in the gap between the Carpathians and the Vistula River. The Russian 3rd Army holding the sector was woefully ill-prepared. A four-hour artillery bombardment destroyed poorly constructed trenches and drove the Russian infantry into headlong flight. Neither the Russian system of command nor their railroad network was capable of a rapid movement of reserves to block the breakthrough. By 10 May, the Russians had retreated to the San River, which was crossed by Austro-German forces a week later. Tens of thousands of Russian soldiers, reduced to fighting with bayonets due to lack of ammunition, surrendered. Russian counterattacks failed and on 3 June Przemysl was retaken by Austria-Hungary. The retreat of the Russian 3rd Army forced the armies to its south to pull back as well. By early July, most of Galicia was in the hands of the Central Powers.