World War I: The German 12th Army occupies Warsaw during the Gorlice–Tarnów Offensive and the Great Retreat of 1915.

The 12th Army was an army level command of the German Army in World War I formed in August 1915 by the redesignation of Armee-Gruppe Gallwitz. It served exclusively on the Eastern Front and was dissolved on 9 October 1916 when its commander, General der Infanterie Max von Fabeck, was transferred to 8th Army.

Warsaw, officially the Capital City of Warsaw, is the capital and largest city of Poland. The metropolis stands on the River Vistula in east-central Poland. Its population is officially estimated at 1.86 million residents within a greater metropolitan area of 3.27 million residents, which makes Warsaw the 6th most-populous city in the European Union. The city area measures 517 km² (200 sq mi) and comprises 18 districts, while the metropolitan area covers 6,100 km² (2,355 sq mi). Warsaw is classified as an alpha global city, a major political, economic and cultural hub, and the country's seat of government. It is also the capital of the Masovian Voivodeship.

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.

The Great Retreat was a strategic withdrawal and evacuation on the Eastern Front of World War I in 1915. The Imperial Russian Army gave up the salient in Galicia and the Polish Congress Kingdom. The Russian Empire's critically under-equipped military suffered great losses in the Central Powers' July–September summer offensive operations, which led to the Stavka ordering a withdrawal to shorten the front lines and avoid the potential encirclement of large Russian forces in the salient. While the withdrawal itself was relatively well-conducted, it was a severe blow to Russian morale.