On September 17, 1939, Soviet troops attacked Poland, and German soldiers retreated to the line formed by the Narev and Vistula rivers.
Germany thus fulfilled its part of the secret agreement that complemented the Non-Aggression Pact signed between Adolf Hitler and Joseph Stalin on August 23, 1939.
The complementary agreement, which became known only much later, provided for the division of Poland between Germany and the Soviet Union, in case of a political and territorial reorganization of Western Europe after the war. While Finland, Estonia, Latvia, and Bessarabia (now Moldova) would be under Soviet rule, the current city of Gdansk would pass to Germany.
The Germans had invaded Poland on September 1, in a massive military attack that became known around the world by the expression blitzkrieg (lightning war). Using the argument that it needed to protect its borders, the Soviet Union attacked from the east.
An unequal share
When marching into Poland on September 17, Stalin’s troops met with little resistance, as the Germans had destroyed much of the Polish armed forces.
Germany eventually took over the rich industrial regions in the western part of the country, and the Soviet Union took the remaining third of the territories in the east. The unequal division did not bother the Soviet leader, as he counted on German support in the attacks on Finland and the Baltic countries (although Germany also showed interest in the city of Vilnius, Lithuania).
Alliances with totalitarian regimes in Rome and Moscow, in addition to the success of the Polish campaign, encouraged Berlin to advance, but this time over Western Europe. The alliance between Hitler and Stalin finally ended on June 22, 1941, when the German Army invaded the Soviet Union.
source: with DW