The Nazi-Soviet pact was a non-aggression agreement entered into between Germany and the Soviet Union whereby they divided eastern Europe into spheres of influence. Each agreed not to interfere in the others area of influence. Germany was given rights to Poland as well as Lithuania; and agreed not to intervene if/when the Soviets attacked Estonia, Latvia and Finland.
The beauty of the agreement for Germany was that it meant that in the event of war, Germany would only have to fight on one front. It would only have to deal with France and England. Further, Hitler believed that if the Western powers believed that the Soviets would not intervene, he might be able to force them to back down as he had done in previous meetings with Neville Chamberlain, the British Prime Minister. The beauty for the Soviets was that it gave them a free hand in Eastern Europe. The Western powers had made no attempt to hide their disdain of dealing with the Communists, and a treaty with the Germans freed the Soviets from becoming isolated.
Many historians believe that the pact was the immediate cause of the war; as Hitler attacked Poland as soon as he was assured that the Soviets would not intervene. It proved disastrous for the Soviets, as Hitler was able to conquer much of Europe before he attacked the Soviet Union. In effect, he managed to outsmart the Soviet leader, Joseph Stalin who never believed that Hitler would attack until the attack came. The attack ultimately forced the Soviets into an uneasy alliance with the Western powerrs which ultimately defeated the Germans.
The Nazi-Soviet Pact, concluded in August of 1939, was a "Nonaggression" agreement between Germany and the Soviet Union, in which both countries recognized the other's "Sphere of Influence" within Europe.
However, one of its secret clauses was the division of Poland between the two countries; historians now see it as a means by which Hitler and Stalin bought time to prepare for war. Once Hitler and Stalin were assured of each's "neutral" intent, the Soviet Union attacked Finland, and Germany invaded Poland. These conflicts then brought England and France into the conflict, and precipitated the widespread outbreak of World War II in Europe.