When Germany invaded Poland, the Soviet leadership adopted a neutral stance. Two weeks after the start of the invasion, the German Army approached the eastern districts of Poland. As the USSR was interested in the security of the Ukrainian and Belorussian population in Polish territories, it could not allow Germany to seize Western Ukraine and Western Belorussia.

On September 17, 1939, Red Army troops crossed the Soviet-­Polish border, penetrating into Western Ukraine and Western Belorussia, which had been seized by Poland during the Polish-Soviet War of 1919–1921.

On October 22, 1939, parliamentary elections to the People’s Assemblies of Western Ukraine and Western Belorussia were held on the basis of universal, equal and direct suffrage using a secret ballot. Voter turnout amounted to almost 93% in Western Ukraine and over 96% in Western Belorussia. In late October 1939, the elected deputies unanimously adopted a declaration on the instauration of Soviet power in Western Ukraine and Western Belorussia and sent a petition to the Supreme Soviet of the USSR for reuniting the Ukrainian and Belorussian nations.

On November 1–2, 1939, the Supreme Soviet of the USSR voted to incorporate Western Ukraine and Western Belorussia into the USSR and reunite them with the Ukrainian and Belorussian SSR. This was a true milestone in the history of the three brotherly Slavic nations.