Lwów Voivodeship (Polish: Województwo lwowskie) was an administrative unit of interwar Poland (1918–1939). Because of the Nazi-Soviet invasion of Poland in accordance with the secret Molotov–Ribbentrop Pact, it became occupied by both the Wehrmacht and the Red Army in September 1939. Following the conquest of Poland however, the Polish underground administration existed there until August 1944. The Voivodeship was not returned to Poland after the war ended. It was split diagonally just east of Przemyśl; with its eastern half ceded to the Ukrainian SSR at the insistence of Joseph Stalin during the Tehran Conference confirmed (as not negotiable) at the Yalta Conference of 1945.