World War I removed former imperial borders across Europe. In 1918, after the Russian Revolution had renounced Tsarist claims to Poland in the Treaty of Brest-Litovsk and the war had ended with Germany's surrender, Poland was able to re-establish its independence after a century of foreign rule.
The Russian Civil War presented an opportunity for Poland under the leadership of Józef Piłsudski to regain parts of the tsarist territories of the former Polish-Lithuanian Commonwealth which had been incorporated into the Russian Empire during the Partitions of Poland. Meanwhile, many in the Soviet leadership desired to respond to Piłsudski's moves into the Ukraine by using military force against Poland, which was seen by the Soviets as a land bridge to Western Europe, and thus to extend the revolution westwards. The Polish–Soviet War ensued, culminating in the Polish victory in the Battle of Warsaw (1920), after which both sides became receptive to ending the conflict. Further military setbacks following their defeat near Warsaw made the Soviets eager to begin peace treaty negotiations, and the Poles, pressured by the League of Nations, were also willing to negotiate after the Polish army had annexed most of the disputed territories in the war, but was nearing exhaustion.