On 1 September 1939, the German invasion of its agreed upon portion of Poland started the Second World War. On 17 September the Red Army invaded eastern Poland and occupied the Polish territory assigned to it by the Molotov–Ribbentrop Pact, followed by co-ordination with German forces in Poland. Eleven days later, the secret protocol of the Molotov–Ribbentrop Pact was modified, allotting Germany a larger part of Poland, while ceding most of Lithuania to the Soviet Union. The Soviet portions lay east of the so-called Curzon Line, an ethnographic frontier between Russia and Poland drawn up by a commission of the Paris Peace Conference in 1919.