To allay Russian pressure on the Austro-Hungarians on the Eastern Front, and to inflict Russia a decisive blow, the German Chief of Staff, Erich von Falkenhayn, had decided for a major offensive in the Gorlice-Tarnów area, south-east of Cracow. Thus, in April 1915, the recently formed German XI Army (10 infantry divisions under General August von Mackensen) was transferred from the Western Front. Along with the Austrian IV Army (8 infantry and 1 cavalry divisions under Archduke Joseph Ferdinand), it had to cope with the Russian III Army (18½ infantry and 5½ cavalry divisions, under General D.R.Radko-Dmitriev), that held that sector.

General Mackensen had been given command of both German and Austro-Hungarian forces, and on May 1, after a heavy artillery bombardment, he launched an attack which caught the Russians by surprise. He concentrated 10 infantry and 1 cavalry division (126,000 men, 457 light, 159 heavy pieces of artillery and 96 mortars) on the 35 km of the breakthrough sector of the frontline against 5 Russian divisions (60,000 men with 141 light and 4 heavy pieces of artillery). Russian defenses were shattered and their lines collapsed. The III Army left in enemy hands about 140,000 prisoners and almost ceased to exist as a fighting unit.

The Russians were forced to withdraw, the Central Powers recaptured most of Galicia, and the Russian threat to Austria-Hungary was averted. Under pressure from the Kaiser, Falkenhayn gave in to Hindenburg and Ludendorff's insistence and widened the eastward offensive. The Germans advanced 150 kilometers (95 miles) in two weeks, expanding the front. Although the Russians, in rear-guard fightings, inflicted severe casualties on the Germans on the San and Dniester rivers, they had not enough resources to halt them. They had to evacuate Przemyśl on June 3, and the Galician capital Lviv (in German: Lemberg; in Polish: Lwów; in Russian: Lvov) on the 22^nd. Between June 23 and 27 the Germans crossed the Dniester.

The Germans, after having received considerable reinforcements, took Brest-Litovsk (about 190 kilometers or 120 miles east of Warsaw) on August 25. With the continuing Russian retreat, the Polish capital itself became isolated, and the German XII Army (under Gallwitz) seized the opportunity and conquered it on August 4–5.

On September 19, Hindenburg's forces captured Vilnius, the capital of Lithuania.

At this point, the German advance was finally halted by bad weather. The new front line now ran 290 kilometers (180 miles) east of Warsaw for almost 1,000 kilometers (600 miles), from the Baltic Sea to the Romanian border.

The Russians had had to evacuate Galicia and lost Poland, suffering heavy casualties. On September 8, Nicholas II personally took command of the Russian armed forces, replacing Grand Duke Nicholas Nikolayevich. However, in spite of the severe defeat, the Russian retreat had been in good order, and Russia was not knocked out of the war. Furthermore, in order to sustain a large-scale offensive, Falkenhayn had had to transfer forces from the Western Front, weakening his chances against the Franco-British, without being able to obtain a decisive victory against the Russians.

1915 | Battles of World War I | Battles of Russia | Battles of Germany