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Background

The war's main territories of contention lie in what is now Ukraine and Belarus. Until the mid-13th century, they formed part of the medieval state of Kievan Rus'. After a period of internal wars and the 1240 Mongol invasion, the lands became objects of expansion for the Kingdom of Poland and for the Grand Duchy of Lithuania. In the first half of the 14th century, the Principality of Kiev and the land between the Dnieper, Pripyat, and Daugava (Western Dvina) rivers became part of the Grand Duchy of Lithuania. In 1352, Poland and Lithuania divided the Kingdom of Galicia–Volhynia between themselves. In 1569, in accordance with the terms of the Union of Lublin between Poland and Lithuania, some of the Ukrainian lands passed to the Polish Crown. Between 1772 and 1795, many of the East Slavic territories became part of the Russian Empire in the course of the Partitions of Poland–Lithuania. In 1795 (the Third Partition of Poland), Poland lost formal independence. After the Congress of Vienna of 1814–1815, much of the territory of the Duchy of Warsaw was transferred to Russian control and became the autonomous Congress Poland (officially the Kingdom of Poland). After young Poles refused conscription to the Imperial Russian Army during the January Uprising of 1863, Tsar Alexander II stripped Congress Poland of its separate constitution, attempted to force general use of the Russian language and took away vast tracts of land from Poles. Congress Poland was incorporated more directly into imperial Russia by being divided into ten provinces, each with an appointed Russian military governor and all under complete control of the Russian Governor-General at Warsaw.

In the aftermath of World War I, the map of Central and Eastern Europe changed drastically. The German Empire 's defeat rendered obsolete Berlin 's plans for the creation of Eastern European German-dominated states (Mitteleuropa), which included another rendition of the Kingdom of Poland. The Russian Empire collapsed, which resulted in the Russian Revolution and the Russian Civil War. The Russian state lost territory due to the German offensive and the Treaty of Brest-Litovsk, signed by the emergent Russian Soviet Federative Socialist Republic. Several nations of the region saw a chance for independence and seized their opportunity to gain it. The defeat of Germany on the Western Front and the withdrawal of the Imperial German Army in the Eastern Front had left Berlin in no position to retaliate against Soviet Russia, which swiftly repudiated the treaty and proceeded to recover many of the former territories of the Russian Empire. However, preoccupied with the civil war, it did not have the resources to react swiftly to the national rebellions.

In November 1918, Poland became a sovereign state. Among the several border wars fought by the Second Polish Republic was the successful Greater Poland uprising (1918–1919) against Weimar Germany. The historic Polish–Lithuanian Commonwealth included vast territories in the east. They had been incorporated into the Russian Empire in 1772–1795 and had remained its parts, as the Northwest Territory, until World War I. After the war they were contested by the Polish, Russian, Ukrainian, Belarusian, Lithuanian, and Latvian interests.

In newly independent Poland, politics were strongly influenced by Józef Piłsudski. On 11 November 1918, Piłsudski was made head of Polish armed forces by the Regency Council of the Kingdom of Poland, a body installed by the Central Powers. Subsequently, he was recognized by many Polish politicians as temporary chief of state and exercised in practice extensive powers. Under the Small Constitution of 20 February 1919, he became chief of state. As such, he reported to the Legislative Sejm.

With the collapse of the Russian and German occupying authorities, virtually all of Poland's neighbours began fighting over borders and other issues. The Finnish Civil War, the Estonian War of Independence, the Latvian War of Independence, and the Lithuanian Wars of Independence were all fought in the Baltic Sea region. Russia was overwhelmed by domestic struggles. In early March 1919, the Communist International was established in Moscow. The Hungarian Soviet Republic was proclaimed in March and the Bavarian Soviet Republic in April. Winston Churchill, in a conversation with Prime Minister David Lloyd George, commented sarcastically: "The war of giants has ended, the wars of the pygmies begin." The Polish–Soviet War was the longest lasting of the international engagements.

The territory of what had become Poland had been a major battleground during World War I and the new country lacked political stability. It had won the hard-fought Polish–Ukrainian War against the West Ukrainian People's Republic by July 1919 but had already become embroiled in new conflicts with Germany (the 1919–1921 Silesian Uprisings) and the January 1919 border conflict with Czechoslovakia. Meanwhile, Soviet Russia focused on thwarting the counterrevolution and the 1918–1925 intervention by the Allied powers. The first clashes between Polish and Soviet forces occurred in autumn and winter 1918/1919, but it took a year and a half for a full-scale war to develop.

The Western powers considered any significant territorial expansion of Poland, at the expense of Russia or Germany, to be highly disruptive to the post-World War I order. Among other factors, the Western Allies did not want to give Germany and Russia a reason to conspire together. The rise of the unrecognized Bolshevik regime complicated this rationale.

The Treaty of Versailles, signed on 28 June 1919, regulated Poland's western border. The Paris Peace Conference (1919–1920) had not made a definitive ruling in regard to Poland's eastern border but on 8 December 1919, the Allied Supreme War Council issued a provisional boundary (its later version would be known as the Curzon Line). It was an attempt to define the areas that had an "indisputably Polish ethnic majority". The permanent border was contingent on the Western powers' future negotiations with White Russia, presumed to prevail in the Russian Civil War. Piłsudski and his allies blamed Prime Minister Ignacy Paderewski for this outcome and caused his dismissal. Paderewski, embittered, withdrew from politics.

The leader of Russia's new Bolshevik government, Vladimir Lenin, aimed to regain control of the territories abandoned by Russia in the Treaty of Brest-Litovsk in March 1918 (the treaty was annulled by Russia on 13 November 1918) and to set up Soviet governments in the emerging countries in the western parts of the former Russian Empire. The more ambitious goal was to also reach Germany, where he expected a socialist revolution to break out. By the end of summer 1919, the Soviets had taken over most of eastern and central Ukraine (formerly parts of the Russian Empire) and driven the Directorate of Ukraine from Kiev. In February 1919, they set up the Socialist Soviet Republic of Lithuania and Belorussia (Litbel). It is however unlikely that the Soviet forced plannes further incursions westward.

From late 1919, Lenin, encouraged by the Red Army's civil war victories over the White Russian forces and their Western allies, began to envision the future of world revolution with greater optimism. The Bolsheviks proclaimed the need for the dictatorship of the proletariat and agitated for a worldwide communist community. They intended to link the revolution in Russia with a communist Revolutions and interventions in Hungary (1918–1920) they had hoped for and to assist other communist movements in Europe. To be able to provide direct physical support to revolutionaries in the West, the Red Army would have to cross the territory of Romania.

According to the historian Andrzej Chwalba, however, the scenario was different in late 1919 and winter–spring 1920. The Soviets, facing decreasing revolutionary fervor in Europe and having to deal with Russia's own problems, attempted to make peace with its neighbors, including Poland.

According to Aviel Roshwald, (Piłsudski) "hoped to incorporate most of the territories of the defunct Polish–Lithuanian Commonwealth into the future Polish state by structuring it as the Polish-led, multinational federation." Piłsudski had wanted to break up the Russian Empire and set up the Intermarium federation of various different states: Poland, Lithuania, Belarus, Ukraine and other Central and East European countries that emerged from the crumbling empires after World War I. In Piłsudski's vision, Poland would replace a truncated and vastly reduced Russia as the great power of Eastern Europe. His plan excluded negotiations prior to military victory. He had hoped that the new Poland-led union would become a counterweight to any potential imperialist intentions of Russia or Germany. Piłsudski believed that there could be no independent Poland without a Ukraine free of Russian control, thus his main interest was in splitting Ukraine from Russia. He used military force to expand the Polish borders in Galicia and Volhynia and crush a Ukrainian attempt at self-determination in the disputed territories east of the Curzon Line, which contained a significant Polish minority. On 7 February 1919, Piłsudski spoke on the subject of Poland's future frontiers:

"At the moment Poland is essentially without borders and all that we can gain in this regard in the west depends on the Entente – on the extent to which it may wish to squeeze Germany. In the east, it's a different matter; there are doors here that open and close and it depends on who forces them open and how far".

Polish military forces had thus set out to expand far in the eastern direction. As Piłsudski imagined,

"Closed within the boundaries of the 16th century, cut off from the Black Sea and Baltic Sea, deprived of land and mineral wealth of the South and South-east, Russia could easily move into the status of second-grade power. Poland, as the largest and strongest of the new states, could easily establish a sphere of influence stretching from Finland to the Caucasus ".

Piłsudski's concepts appeared more progressive and democratic in comparison with the rival National Democracy 's plans, although both pursued the idea of direct incorporation and Polonization of the disputed eastern lands. However Piłsudski used his "federation" idea instrumentally. As he wrote to his close associate Leon Wasilewski in April 1919, (for now)

"I want to be neither an imperialist nor a federalist.... Taking into account that, in this God's world, an empty talk of the brotherhood of people and nations as well as the American little doctrines seem to be winning, I gladly side with the federalists".

According to Chwalba, the differences between Piłsudski's vision of Poland and that of his rival National Democratic leader Roman Dmowski were more rhetorical than real. Piłsudski had made many obfuscating statements, but never specifically stated his views regarding Poland's eastern borders or political arrangements he intended for the region.