Pilsudski's Kiev thrust and the Peace of Riga redrew the east
The Polish–Soviet War ran from 14 February 1919 to 18 March 1921 mainly between the Second Polish Republic and Soviet Russia after World War I and revolution. Lenin annulled Brest-Litovsk and pushed west once the Central Powers collapsed in November 1918.
The contested ground, today's Ukraine and Belarus, had belonged to Kievan Rus' until the Mongol invasion of 1240, then drew Polish and Lithuanian expansion; in 1352 the two powers carved up Galicia–Volhynia between them. Those eastern lands passed into the Russian Empire during 1772–1795 and stayed there until the First World War. When Poland regained sovereignty in November 1918, it fought several border wars at once, including a successful rising in Greater Poland against Weimar Germany.
Each side had grand designs. Lenin saw Poland as the corridor through which revolution could reach Europe and sent troops into the Ober Ost zone the Germans had vacated. Józef Piłsudski wanted frontiers resembling those before 1772 and dreamed of an Intermarium federation led by Warsaw. Through 1919 Polish forces took much of present-day Lithuania and Belarus and won their war against the Ukrainians.
In April 1920 Piłsudski struck at Ukraine, and on 7 May Polish and allied Ukrainian troops entered Kiev. The Red Army had not been beaten, however, and local enthusiasm was thin: many Ukrainians enlisted with the Soviets rather than with Petliura. A Soviet counterattack from June drove the Poles back to Warsaw by August, where an unexpected Polish victory between 12 and 25 August reversed the war.
A ceasefire took effect on 18 October 1920, which is why some accounts end the war that year, while the Peace of Riga was signed only on 18 March 1921. It put Poland's eastern border about 200 kilometres beyond the Curzon Line, handing it parts of modern Ukraine and Belarus, but it also confirmed Soviet Ukraine and Soviet Belarus, burying the Intermarium idea. Poles often call the conflict the Bolshevik War or simply the War of 1920.
Source: Polish–Soviet War
