The Polish–Soviet War (14 February 1919 – 18 March 1921) was fought primarily between the Second Polish Republic and the Russian Soviet Federative Socialist Republic, following World War I and the Russian Revolution.
After the collapse of the Central Powers and the Armistice of 11 November 1918, Vladimir Lenin's Soviet Russia annulled the Treaty of Brest-Litovsk and moved forces westward to reclaim the Ober Ost regions abandoned by the Germans. Lenin viewed the newly independent Poland as a critical route for spreading communist revolutions into Europe. Meanwhile, Polish leaders, including Józef Piłsudski, aimed to restore Poland's pre-1772 borders and secure the country's position in the region.