In the aftermath of World War I, the map of Central and Eastern Europe had drastically changed. Germany's defeat rendered its plans for the creation of Eastern European puppet states (Mitteleuropa) obsolete, and the Russian Empire collapsed, resulting in a revolution and a civil war. Many small nations of the region saw a chance for real independence and seized the opportunity to gain it; Soviet Russia viewed these territories as rebellious Russian provinces, vital for Russian security, but was unable to react swiftly. While the Paris Peace Conference had not made a definitive ruling in regard to the Poland's eastern border, it issued a provisional boundary in December 1919 - the Curzon line - as an attempt to define the territories that had an "indisputably Polish ethnic majority"; the participants did not feel competent to make a certain judgment on the competing claims.
With the success of the Greater Poland Uprising in 1918, Poland had re-established its statehood for the first time since the 1795 partition and seen the end of a 123 years of rule by three imperial neighbors: Russia, Germany, and Austria-Hungary. The country, reborn as a Second Polish Republic, proceeded to carve out its borders from the territories of its former partitioners.
Poland was not alone in its newfound opportunities and troubles. With the collapse of Russian and German occupying authorities, virtually all of the newly independent neighbours began fighting over borders: Romania fought with Hungary over Transylvania, Yugoslavia with Italy over Rijeka, Poland with Czechoslovakia over Cieszyn Silesia, with Germany over Poznań and with Ukrainians over Eastern Galicia. Ukrainians, Belarusians, Lithuanians, Estonians and Latvians fought against each other and against the Russians, who were just as divided. Spreading Communist influences resulted in Communist revolutions in Munich, Berlin, Budapest and Prešov. Winston Churchill commented: "The war of giants has ended, the wars of the pygmies began." All of these engagements – with the sole exception of the Polish-Soviet war – would be short-lived.
Vladimir Lenin, was inspired by the Red Army's civil-war victories over White Russian anti-communist forces and their Western allies, and began to see the future of the revolution with greater optimism. The Bolsheviks proclaimed the need for the dictatorship of the proletariat, and agitated for a worldwide Communist community. Their avowed intent was to link the revolution in Russia with an expected revolution in Germany and to assist other Communist movements in Western Europe; Poland was the geographical bridge that the Red Army would have to cross in order to do so. Lenin’s aim was to regain control of the territories ceded by Russia in the Brest-Litovsk Treaty, to infiltrate the borderlands, set up Soviet governments there as well as in Poland, and reach Germany where he expected a Socialist revolution to break out. He believed that Soviet Russia could not survive without the support of a socialist Germany. By the end of summer 1919 the Soviets managed to take over most of Ukraine, driving the Ukrainian Directorate from Kiev. In early 1919, they also set up a Lithuanian-Belorussian Republic (Litbel). This government was very unpopular due to terror and the collection of food and goods for the army. It was not until after the Polish Kiev Offensive had been repelled, however, that some of the Soviet leaders would see the war as the real opportunity to spread the revolution westwards. Indeed, the Bolsheviks stated:
“ But our enemies and yours deceive you when they say that the Russian Soviet Government wishes to plant communism in Polish soil with the bayonets of Russian Red Army men. A communist order is possible only where the vast majority of the working people are penetrated with the idea of creating it by their own strength. Only then can it be solid; for only then can communist policy strike deep roots in a country. The communists of Russia are at present striving only to defend their own soil, their own constructive work; they are not striving, and cannot strive, to plant communism by force in other countries.”[23] ”
Before the start of the Polish-Soviet War, Polish politics were strongly influenced by Chief of State (naczelnik państwa) Józef Piłsudski. Piłsudski wanted to break up the Russian Empire and create a Polish-led "Międzymorze Federation" of independent states: Poland, Lithuania, Ukraine, and other Central and East European countries emerging out of crumbling empires after the First World War. This new union was to become a counterweight to any potential imperialist intentions on the part of Russia or Germany. Piłsudski argued that "There can be no independent Poland without an independent Ukraine", but he may have been more interested in Ukraine being split from Russia than in Ukrainians' welfare. He did not hesitate to use military force to expand the Polish borders to Galicia and Volhynia, crushing a Ukrainian attempt at self-determination in the disputed territories east of the Western Bug river, which contained a significant Polish minority, mainly in cities like Lviv, but a Ukrainian majority in the countryside. Speaking of Poland's future frontiers, Piłsudski said: "All that we can gain in the west depends on the Entente — on the extent to which it may wish to squeeze Germany," while in the east "there are doors that open and close, and it depends on who forces them open and how far."[34] In the chaos to the east the Polish forces set out to expand there as much as it was feasible. On the other hand, Poland had no intention of joining the western intervention in the Russian Civil War or of conquering Russia itself.