What was the prelude to the Molotov Ribbonov Pact? Why did Germany Reach out to Russia?

Germany had been ignoring the limitations placed on it by the Treaty of Versailles ever since the early to mid 1930's. Hitler had come to power. He had increased the size of the army, begun building the air force and increased the size and potency of the navy. He had sent armed troops into the demilitarized Rhineland in 1936 against the terms of the treaty. After much propaganda he invaded ("liberated") Austria in 1938.

In March 1939 he violated the terms of the Munich agreement and invaded Czechoslovakia. In the spring of 1939 he made demands for the Polish to turn over parts of their country to Germany. After years of bowing to Hitler's demands the British and French finally stood up to him and said that if he attacked Poland they would declare war on him.