On September 1, 1939, the German invasion of its agreed upon portion of western Poland started World War II. On September 17 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.