It was on August 23, 1939 that Adolf Hitler and Joseph Stalin through their representatives Ribbentrop and Molotov signed the German-Soviet Non-Aggression Pact of friendship. It guaranteed Hitler that an invasion of Poland would not result in a war with the USSR. In fact, Stalin took the opportunity in secret provisions to ensure that the Soviet Army could occupy the territories of Galicia in the eastern part of the Polish state where over 4-4.5 million Ukrainians lived. Stalin also agreed to supply Germany with essential war supplies and did so right up until the day Germany invaded the USSR.

On September 1, 1939 Germany invaded Poland and started World War II. From the first day of the war Ukrainians suffered because German bombs killed many Ukrainian civilians in Poland and there were Ukrainians serving in the Polish armed forces. The USSR took this opportunity to add the Ukrainian populated territory of Bukovina from Romania.

Although the Ukrainian Lemkos were left in German occupied Poland, which was renamed the General Government, most of the Ukrainian nation finally was united into one country, the Ukrainian SSR. This was one of the few positive aspects of the war for Ukraine. It also brought together Ukrainians from the western and eastern areas of Ukraine for over a year and a half under Soviet Russian rule until the German invasion of 1941.

However, according to Vasyl Hryshko "During the Bolshevik rule in Western Ukraine (from September 17, 1939 to June 22, 1941) about 750,000 men and women were killed or deported to Siberia." (Hrysko p. 117).