The Soviet Union signed a Pact with Nazi Germany on 23 August 1939 containing a protocol about partition of Poland (generally known but denied by the Soviet Union for the next 50 years). The German army attacked Poland on 1 September 1939. The Soviet Union followed suit by invading eastern Poland on 17 September 1939. Within weeks, 61.2% of Polish Jews found themselves under the German occupation, while 38.8% were trapped in the Polish areas annexed by the Soviet Union. Based on population migration from West to East during and after the German invasion the percentage of Jews under the Soviet-occupation was substantially higher than that of the national census.