German-Soviet Non-Aggression Pactan agreement between Nazi Germany and the Soviet Union on the eve of WWII. The pact was a temporary alliance of adversaries which secured Hitler's eastern front and bought time for Soviet military preparedness. The agreement was breached by Germany's invasion of the Soviet Union less than 2 years later.
The Pact was signed one week before Germany attacked Poland on September 1, 1939. It divided Poland into two spheres of influence with the eastern parts being of interest to the Soviet Union and the western parts being in the sphere of influence of Nazi Germany. The dividing line was along the Narew, Vistula and San rivers.
By a special protocol the Baltic states were recognized as part of the Soviet sphere, with Lithuania's claim to Vilna acknowledged by the Germans. The pact which was supposed to last 10 years was terminated by the invasion of the Soviet Union by Germany on June 22, 1941.
Source: Encyclopedia of the Holocaust.