Lwów (Lviv) was a multicultural city just before World War II, with a population of 312,231. The city's 157,490 ethnic Poles constituted over 50 per cent, with Jews at 32 per cent (99,595) and Ukrainians at 16 per cent (49,747). After the joint Soviet-German invasion of Poland on 1 and 17 September 1939, the USSR and Nazi Germany signed the German–Soviet Frontier Treaty on 28 September 1939, which assigned about 200,000 km2 (77,000 sq mi) of Polish territory inhabited by 13.5 million people of all nationalities to the Soviet Union. Lwów was then annexed to the Soviet Union as part of the Ukrainian SSR.