The Polish–Soviet War was fought primarily between the Second Polish Republic and the Russian Soviet Federative Socialist Republic in the aftermath of World War I, on territories formerly held by the Russian Empire and the Austro-Hungarian Empire.
In newly independent Poland, leading politicians of different orientations pursued the general expectation of restoring the pre-1772 borders. Motivated by that idea, Polish Chief of State Józef Piłsudski aimed to expand Poland's eastern frontiers to the east and began moving troops in that direction.
On 13 November 1918, Vladimir Lenin's Russia annulled the Treaty of Brest-Litovsk and soon started slowly moving forces in the western direction to recover and secure the lands vacated by the German forces that were lost by the Russian state under the treaty. In 1919, while the Red Army was preoccupied with the Russian Civil War, the Polish Army took most of Lithuania and Belarus. Lenin saw Poland as the bridge the Red Army had to cross to assist other communist movements and to bring about more European revolutions.
By July 1919, Polish forces had taken control of much of Western Ukraine and emerged victorious from the Polish–Ukrainian War.