 At first , Hitler rebuffed Soviet diplomatic hints that Stalin desired a treaty ; but in early August 1939 , Hitler authorised Foreign Minister Joachim von Ribbentrop to begin serious negotiations . A trade agreement was concluded on 18 August ; and on 22 August , Ribbentrop flew to Moscow to conclude a formal non @-@ aggression treaty . Although the treaty is known as the Molotov @-@ Ribbentrop Pact , it was Stalin and Hitler , and not Molotov and Ribbentrop , who decided the content of the treaty . The most important part of the agreement was the secret protocol , which provided for the partition of Poland , Finland , and the Baltic States between Nazi Germany and the Soviet Union and for the Soviet annexation of Bessarabia ( then part of Romania , now Moldova ) . This protocol gave Hitler the green light for his invasion of Poland , which began on 1 September . On 5 March 1940 Lavrentiy Beria gave Molotov , along with Anastas Mikoyan , Kliment Voroshilov and Stalin , a note ordering the execution of 25 @,@ 700 Polish officers and anti @-@ Soviets , in what has become known as the Katyn massacre . 
