Why was the Molotov-Ribbentrop pact made?

The Soviet Union had been repeatedly ignored in its attempts to enter into a collective-security agreement with Britain and France against Nazi Germany, most notably at the time of the Munich Conference (September 1938). By early 1939 the Soviets faced the prospect of resisting German military expansion in eastern Europe virtually alone, and so they began searching about for a change of policy. On May 3, 1939, Soviet leader Joseph Stalin fired Foreign Minister Maksim Litvinov, a Jew and an advocate of collective security, and replaced him with V.M. Molotov, who soon began negotiations with the Nazi foreign minister, Joachim von Ribbentrop. The Soviets also kept negotiating with Britain and France, but in the end Stalin chose to reach an agreement with Germany. By doing so he hoped to keep the Soviet Union at peace with Germany and to gain time to build up the Soviet military establishment, which had been badly weakened by the purge of the Red Army officer corps in 1937. The Western democracies' hesitance in opposing Adolf Hitler, along with Stalin's own inexplicable personal preference for the Nazis, also played a part in Stalin's final choice. For his part, Hitler wanted a nonaggression pact with the Soviet Union so that his armies could invade Poland virtually unopposed by a major power, after which Germany could deal with the forces of France and Britain in the west without having to simultaneously fight the Soviet Union on a second front in the east. The end result of the German-Soviet negotiations was the Nonaggression Pact, which was dated August 23 and was signed by Ribbentrop and Molotov in the presence of Stalin, in Moscow.

Source: Britannica Online