After the 1939 invasion of Poland Stryjkowski escaped from Warsaw to Soviet-occupied Lwów (modern Lviv, Ukraine), where he was among the journalists of "Czerwony Sztandar", a Polish language propaganda newspaper published by the Soviets, and the only newspaper available to city's inhabitants apart from the "Pravda". After the end of the Molotov–Ribbentrop Pact and the outbreak of Operation Barbarossa through Tarnopol, Kiev and Stalingrad he escaped to Kuybyshev, where he tried to join the Polish II Corps. Unsuccessful, he moved to Uzbekistan, where he started working in a factory. On insistence of Wanda Wasilewska he was allowed by the Soviet authorities to move to Moscow, where he started working for the "Wolna Polska" weekly, the organ of Society of Polish Patriots, a communist and Soviet-backed shadow government of Poland. There he adopted the pen name of Julian Stryjkowski, which after World War II became his official surname.