linguistic:paraphrasing
Background

Germany and the Soviet Union remained unsatisfied with the outcome of World War I (1914–1918). Soviet Russia had lost substantial territory in Eastern Europe as a result of the Treaty of Brest-Litovsk (March 1918), where the Bolsheviks in Petrograd conceded to German demands and ceded control of Poland, Lithuania, Estonia, Latvia, Finland, and other areas, to the Central Powers. Subsequently, when Germany in its turn surrendered to the Allies (November 1918) and these territories became independent states under the terms of the Paris Peace Conference of 1919 at Versailles, Soviet Russia was in the midst of a civil war and the Allies did not recognise the Bolshevik government, so no Soviet Russian representation attended.

Adolf Hitler had declared his intention to invade the Soviet Union on 11 August 1939 to Carl Jacob Burckhardt, League of Nations Commissioner, by saying:

Everything I undertake is directed against the Russians. If the West is too stupid and blind to grasp this, then I shall be compelled to come to an agreement with the Russians, beat the West and then after their defeat turn against the Soviet Union with all my forces. I need the Ukraine so that they can't starve us out, as happened in the last war.

The Molotov–Ribbentrop Pact signed in August 1939 was a non-aggression agreement between Germany and the Soviet Union. It contained a secret protocol aiming to return Central Europe to the pre–World War I status quo by dividing it between Germany and the Soviet Union. Finland, Estonia, Latvia and Lithuania would return to the Soviet control, while Poland and Romania would be divided. The Eastern Front was also made possible by the German–Soviet Border and Commercial Agreement in which the Soviet Union gave Germany the resources necessary to launch military operations in Eastern Europe.

On 1 September 1939 Germany invaded Poland, starting World War II. On 17 September, the Soviet Union invaded Eastern Poland, and, as a result, Poland was partitioned among Germany, the Soviet Union and Lithuania. Soon after that, the Soviet Union demanded significant territorial concessions from Finland, and after Finland rejected Soviet demands, the Soviet Union attacked Finland on 30 November 1939 in what became known as the Winter War – a bitter conflict that resulted in a peace treaty on 13 March 1940, with Finland maintaining its independence but losing its eastern parts in Karelia.

In June 1940 the Soviet Union occupied and illegally annexed the three Baltic states (Estonia, Latvia and Lithuania). The Molotov–Ribbentrop Pact ostensibly provided security to the Soviets in the occupation both of the Baltics and of the north and northeastern regions of Romania (Northern Bukovina and Bessarabia, June–July 1940), although Hitler, in announcing the invasion of the Soviet Union, cited the Soviet annexations of Baltic and Romanian territory as having violated Germany's understanding of the pact. Moscow partitioned the annexed Romanian territory between the Ukrainian and Moldavian Soviet Socialist Republics.