An armed struggle between the Bolsheviks and Poland resulted from Russian attempts to carry the revolution westward and from Pilsudski's federalist policy. The Great Powers failed to pursue either an all-out intervention against the Bolsheviks or a policy of peace. An Allied proposal for a temporary border between Bolshevik Russia and Poland (called the Curzon Line) was unacceptable to either side. Except for an alliance with the Ukrainian leader Symon Petlyura (April 1920), whose troops accompanied the Poles as they captured Kiev in May, Poland fought in isolation. An offensive by the Red Army drove the Poles back to the outskirts of Warsaw, but the country was saved from catastrophe by Pilsudski's counterattack on August 16 (the "Miracle of the Vistula"). In the compromise Peace of Riga (March 1921), the Bolsheviks abandoned their plans to communize Poland, but the Poles had to abandon their federalist concepts. The new border, which corresponded roughly to the 1793 frontier, cut across mixed Ukrainian and Belarusian territories. In the north it included Wilno, captured by General Lucjan Zeligowski, a move that dug a chasm between Lithuania and Poland.

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Russo-Polish War (1919-20), military conflict between Soviet Russia and Poland, which sought to seize Ukraine. It resulted in the establishment of the Russo-Polish border that existed until 1939.

Although there had been hostilities between the two countries during 1919, the conflict began when the Polish head of state Józef Pilsudski formed an alliance with the Ukrainian nationalist leader Symon Petlyura (April 21, 1920) and their combined forces began to overrun Ukraine, occupying Kiev on May 7. In June the Soviet Red Army launched a counteroffensive, reaching the former Polish border by the end of July. In a wave of revolutionary enthusiasm, Soviet forces advanced through Poland to the outskirts of Warsaw (early August). The western European powers, fearing that the Russians might succeed in establishing a Soviet government in Poland and perhaps proceed to Germany, sent a military mission, headed by the French general Maxime Weygand, to advise the Polish army. The Poles counterattacked in mid-August and forced the Russians to retreat.

An armistice was signed in October 1920. The Treaty of Riga, concluded on March 18, 1921, provided for the bulk of Ukraine to remain a Soviet republic, although substantial portions of Belorussia (Belarus) and Ukraine were ceded to Poland.