After the Soviet invasion of Poland of 1939, Borejsza became a vocal supporter of the Soviet communist regime, publishing Polish language translations of Soviet propaganda. He served as director of the Ossolineum Institute in Lwów (Lviv) in 1939–1940. After the war, as Lviv was transferred to the Ukrainian SSR, he aided the transport of most of Ossolineum archives to Wrocław. He was one of the founders of the Union of Polish Patriots – an organization from which the communist government of post-war Poland in part originated. Borejsza served with the rank of major in the Red Army, and then in the Polish First Army.