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Civil War and the Polish–Soviet War: 1918–1920

Following the Armistice and the withdrawal of German Ober Ost garrisons, Soviet and Polish armies moved to fill the vacuum. Both Soviet Russia and the new Polish state sought territorial expansion. Polish and Russian troops first clashed in February 1919, escalating into the Polish–Soviet War. Unlike previous conflicts, this war had significant implications for the export of revolution and Europe's future. Polish forces advanced into Ukraine, taking Kiev by May 1920. After forcing the Polish Army back, Lenin urged the Red Army to invade Poland, expecting a proletarian uprising that would ignite a European revolution. Despite scepticism from Trotsky and others, the invasion proceeded, but the Polish proletariat did not rise, and the Red Army was defeated at the Battle of Warsaw. The Polish Army pushed the Red Army back into Russia, forcing Sovnarkom to sue for peace, culminating in the Peace of Riga, where Russia ceded territory to Poland.